drm/virtio: fix command submission with objects but without fence.

Only call virtio_gpu_array_add_fence if we actually have a fence.

Fixes: da758d5 ("drm/virtio: rework virtio_gpu_execbuffer_ioctl fencing")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190904074828.32502-1-kraxel@redhat.com

drivers/gpu/drm/virtio/virtgpu_vq.c

@@ -339,11 +339,12 @@ static void virtio_gpu_queue_fenced_ctrl_buffer(struct virtio_gpu_device *vgdev,
 		goto again;
 	}
 
-	if (fence)
+	if (fence) {
 		virtio_gpu_fence_emit(vgdev, hdr, fence);
-	if (vbuf->objs) {
-		virtio_gpu_array_add_fence(vbuf->objs, &fence->f);
-		virtio_gpu_array_unlock_resv(vbuf->objs);
+		if (vbuf->objs) {
+			virtio_gpu_array_add_fence(vbuf->objs, &fence->f);
+			virtio_gpu_array_unlock_resv(vbuf->objs);
+		}
 	}
 	notify = virtio_gpu_queue_ctrl_buffer_locked(vgdev, vbuf);
 	spin_unlock(&vgdev->ctrlq.qlock);