drm/sched: add missing member documentation

Just fix a warning.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: f2f12eb9 ("drm/scheduler: provide scheduler score externally")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210401125213.138855-1-christian.koenig@amd.com

include/drm/gpu_scheduler.h

@@ -277,6 +277,7 @@ struct drm_sched_backend_ops {
  * @hang_limit: once the hangs by a job crosses this limit then it is marked
  *              guilty and it will be considered for scheduling further.
  * @score: score to help loadbalancer pick a idle sched
+ * @_score: score used when the driver doesn't provide one
  * @ready: marks if the underlying HW is ready to work
  * @free_guilty: A hit to time out handler to free the guilty job.
  *