radv + steamvr

If anyone wants to run SteamVR on top of radv, the code is all public now.

https://github.com/airlied/mesa/tree/radv-wip-steamvr

The external memory code will be going upstream to master once I clean it up a bit, the semaphore hack is waiting on kernel
changes, and the NIR shader hack is waiting on a new SteamVR build that removes the bad use of SPIR-V.

I've run Serious SAM TFE in VR mode on this branch.