drm/virtio: call drm_plane_cleanup() at destroy phase

virtio was missing this call to clean up core plane usage.
Signed-off-by: NGustavo Padovan <gustavo.padovan@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215180009.7091-1-gustavo@padovan.orgSigned-off-by: NGerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -44,6 +44,7 @@ static const uint32_t virtio_gpu_cursor_formats[] = {
 
 static void virtio_gpu_plane_destroy(struct drm_plane *plane)
 {
+	drm_plane_cleanup(plane);
 	kfree(plane);
 }