drm/virtio: call drm_plane_cleanup() at destroy phase

virtio was missing this call to clean up core plane usage.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215180009.7091-1-gustavo@padovan.org


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>