drm/amdgpu: set the VM pointer to NULL in amdgpu_job_prepare

As soon as the prepare phase is completed the VM might be released,
better set it to NULL.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -361,6 +361,13 @@ amdgpu_job_prepare_job(struct drm_sched_job *sched_job,
 			dev_err(ring->adev->dev, "Error getting VM ID (%d)\n", r);
 			goto error;
 		}
+		/*
+		 * The VM structure might be released after the VMID is
+		 * assigned, we had multiple problems with people trying to use
+		 * the VM pointer so better set it to NULL.
+		 */
+		if (!fence)
+			job->vm = NULL;
 	}
 
 	return fence;