winsys/amdgpu: Add amdgpu_screen_winsys

winsys/amdgpu: Add amdgpu_screen_winsys

It extends pipe_screen / radeon_winsys and references amdgpu_winsys.
Multiple amdgpu_screen_winsys instances may reference the same
amdgpu_winsys instance, which corresponds to an amdgpu_device_handle.

The purpose of amdgpu_screen_winsys is to keep a duplicate of the DRM
file descriptor passed to amdgpu_winsys_create, which will be needed
in the next change.

v2:
* Add comment in amdgpu_winsys_unref explaining why it always returns
  true (Marek Olšák)

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>