UPSTREAM: drm/virtio: add missing drm_atomic_helper_shutdown() call.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190401140306.28063-2-kraxel@redhat.com


(cherry picked from commit 7ff8a316d10b7eca9dbf4bd6aaedb0db1757139d)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: Ic4c2eb5bb4de639fc9fbae55b8848299acfe5f14