amdgpu: Add new function to get fd.
Dual purpose:
- The drm fd dedupe functionality confuses the radeonsi amdgpu winsys if radeonsi isn't the first thing opening the device. By exposing the fd we can detect this case.
- For a common mesa Vulkan sync objects implementation with syncobj. (notable: no buffer allocation)
Both shouldn't interferece with libdrm_amdgpu functionality though it does somewhat piece the abstraction of the library.
Signed-off-by: Bas Nieuwenhuizen bas@basnieuwenhuizen.nl Gitlab: mesa#3424 (closed) Gitlab: mesa#5630 (closed)