winsys/amdgpu: Only re-export KMS handles for different DRM FDs

winsys/amdgpu: Only re-export KMS handles for different DRM FDs

When the amdgpu_screen_winsys uses the same FD as the amdgpu_winsys
(which is always the case for the first amdgpu_screen_winsys), we can
just use bo->u.real.kms_handle.

v2:
* Also only create the kms_handles hash table if the
  amdgpu_screen_winsys fd is different from the amdgpu_winsys one.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3202>