winsys/amdgpu: Re-use amdgpu_screen_winsys when possible

winsys/amdgpu: Re-use amdgpu_screen_winsys when possible

Namely, if os_same_file_description determined that the DRM file
descriptor references the same file description.

v2:
* Adapt to amdgpu_winsys::sws_list_lock.
v3:
* Fix comparison of amdgpu_screen_winsys file descriptions, see
  https://gitlab.freedesktop.org/mesa/mesa/issues/2413 .
* Lock amdgpu_winsys::sws_list_lock for traversing the sws_list in
  amdgpu_winsys_create.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3582>