drm/amdgpu: Use DRM scheduler API in amdgpu_xcp_release_sched

Lets use the existing helper instead of peeking into the structure
directly.

Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Tvrtko Ursulin <[e-mail]>
Cc: Christian König <[e-mail]>
Cc: Danilo Krummrich <[e-mail]>
Cc: Matthew Brost <[e-mail]>
Cc: Philipp Stanner <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_xcp.c

···
 		return;
 
 	sched = entity->entity.rq->sched;
-	if (sched->ready) {
+	if (drm_sched_wqueue_ready(sched)) {
 		ring = to_amdgpu_ring(entity->entity.rq->sched);
 		atomic_dec(&adev->xcp_mgr->xcp[ring->xcp_id].ref_cnt);
 	}