Upgraded graphics card RX 470 4GiB -> RX 580 8GiB, available system ram reduced from 16 GiB to 8 GiB

As the title reads.

The motherboard is a Gigabyte GA-990X-Gaming SLI.

I already had to enable IOMMU and pass to the kernel the option iommu=pt to get the integrated NIC and USB ports to work.

I upgraded the graphics card to one with bigger VRAM, from 4GiB to 8GiB. But now the system shows half the RAM.

The boot log of GNU/Linux shows this:

[ 0.000000] Faking a node at [mem 0x0000000000000000-0x000000023effffff]
[ 0.000000] NODE_DATA(0) allocated [mem 0x23efd5000-0x23effffff]
[ 0.000000] tsc: Fast TSC calibration using PIT
[ 0.000000] Zone ranges:
[ 0.000000] DMA [mem 0x0000000000001000-0x0000000000ffffff]
[ 0.000000] DMA32 [mem 0x0000000001000000-0x00000000ffffffff]
[ 0.000000] Normal [mem 0x0000000100000000-0x000000023effffff]
[ 0.000000] Device empty
[ 0.000000] Movable zone start for each node
[ 0.000000] Early memory node ranges
[ 0.000000] node 0: [mem 0x0000000000001000-0x000000000009dfff]
[ 0.000000] node 0: [mem 0x0000000000100000-0x00000000be6b4fff]
[ 0.000000] node 0: [mem 0x00000000bf14f000-0x00000000bf14ffff]
[ 0.000000] node 0: [mem 0x00000000bf356000-0x00000000bf7fffff]
[ 0.000000] node 0: [mem 0x0000000100001000-0x000000023effffff]
[ 0.000000] Reserved but unavailable: 5380 pages
[ 0.000000] Initmem setup node 0 [mem 0x0000000000001000-0x000000023effffff]
[ 0.000000] On node 0 totalpages: 2087676
[ 0.000000] DMA zone: 64 pages used for memmap
[ 0.000000] DMA zone: 21 pages reserved
[ 0.000000] DMA zone: 3997 pages, LIFO batch:0
[ 0.000000] DMA32 zone: 12142 pages used for memmap
[ 0.000000] DMA32 zone: 777056 pages, LIFO batch:31
[ 0.000000] Normal zone: 20416 pages used for memmap
[ 0.000000] Normal zone: 1306623 pages, LIFO batch:31

Previously, the boot log looked like this one:

[ 0.000000] Faking a node at [mem 0x0000000000000000-0x000000043effffff]
[ 0.000000] NODE_DATA(0) allocated [mem 0x43efd3000-0x43effdfff]
[ 0.000000] tsc: Unable to calibrate against PIT
[ 0.000000] tsc: No reference (HPET/PMTIMER) available
[ 0.000000] Zone ranges:
[ 0.000000] DMA [mem 0x0000000000001000-0x0000000000ffffff]
[ 0.000000] DMA32 [mem 0x0000000001000000-0x00000000ffffffff]
[ 0.000000] Normal [mem 0x0000000100000000-0x000000043effffff]
[ 0.000000] Device empty
[ 0.000000] Movable zone start for each node
[ 0.000000] Early memory node ranges
[ 0.000000] node 0: [mem 0x0000000000001000-0x000000000009dfff]
[ 0.000000] node 0: [mem 0x0000000000100000-0x00000000be6b4fff]
[ 0.000000] node 0: [mem 0x00000000bf14f000-0x00000000bf14ffff]
[ 0.000000] node 0: [mem 0x00000000bf356000-0x00000000bf7fffff]
[ 0.000000] node 0: [mem 0x0000000100001000-0x000000043effffff]
[ 0.000000] Initmem setup node 0 [mem 0x0000000000001000-0x000000043effffff]
[ 0.000000] On node 0 totalpages: 4184828
[ 0.000000] DMA zone: 64 pages used for memmap
[ 0.000000] DMA zone: 21 pages reserved
[ 0.000000] DMA zone: 3997 pages, LIFO batch:0
[ 0.000000] DMA32 zone: 12142 pages used for memmap
[ 0.000000] DMA32 zone: 777056 pages, LIFO batch:31
[ 0.000000] Normal zone: 53184 pages used for memmap
[ 0.000000] Normal zone: 3403775 pages, LIFO batch:31
[ 0.000000] Reserved but unavailable: 99 pages

The number of unavailable pages has increased and the zone ranges has been halved. Do you know if this can be solved? I have modified things in BIOS to no avail:

1. Boot in UEFI only mode (with IOMMU enabled)
2. Disable IOMMU
3. Boot in legacy only mode (with IOMMU enabled)
4. Disable legacy USB and serial ports, with the hope that the address space was released.

Nothing worked to take the address space back. Physical memory is detected by BIOS and by the kernel, but it remains reserved, unusable.