drm/virtio: add missing virtio_gpu_array_lock_resv call

When submitting a fenced command we must lock the object reservations
because virtio_gpu_queue_fenced_ctrl_buffer() unlocks after adding the
fence.
Reported-by: NJann Horn <jannh@google.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NChia-I Wu <olvaffe@gmail.com>
Tested-by: NJann Horn <jannh@google.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200110094535.23472-1-kraxel@redhat.com

drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -232,6 +232,7 @@ static void virtio_gpu_cursor_plane_update(struct drm_plane *plane,
 		if (!objs)
 			return;
 		virtio_gpu_array_add_obj(objs, vgfb->base.obj[0]);
+		virtio_gpu_array_lock_resv(objs);
 		virtio_gpu_cmd_transfer_to_host_2d
 			(vgdev, 0,
 			 plane->state->crtc_w,