drm/amd/sched: NULL out the s_fence field after run_job

amd_sched_process_job drops the fence reference, so NULL out the s_fence
field before adding it as a callback to guard against accidentally using
s_fence after it may have be freed.

v2: add a clarifying comment
Signed-off-by: NNicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NAndres Rodriguez <andresx7@gmail.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/scheduler/gpu_scheduler.c

@@ -611,6 +611,10 @@ static int amd_sched_main(void *param)
 
 		fence = sched->ops->run_job(sched_job);
 		amd_sched_fence_scheduled(s_fence);
+
+		/* amd_sched_process_job drops the job's reference of the fence. */
+		sched_job->s_fence = NULL;
+
 		if (fence) {
 			s_fence->parent = dma_fence_get(fence);
 			r = dma_fence_add_callback(fence, &s_fence->cb,