So userspace knows when it is available.

Proposed mesa patch:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17411/diffs?commit_id=c8a63590dfd0d64e6e6a634dcfed993f135dd075Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>