drm/amdgpu: add check for callback

it is possible that the callback isn't defined sometimes.
Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian K?nig <christian.koenig@amd.com>

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

@@ -137,6 +137,7 @@ static void amdgpu_job_work_func(struct work_struct *work)
 		container_of(work, struct amdgpu_cs_parser,
 			     job_work);
 	mutex_lock(&sched_job->job_lock);
+	if (sched_job->free_job)
 		sched_job->free_job(sched_job);
 	mutex_unlock(&sched_job->job_lock);
 	/* after processing job, free memory */