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: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3424
Gitlab: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5630


Reviewed-by: Marek Olšák <marek.olsak@amd.com>