drm/nouveau/therm: use workqueue to shutdown the machine

orderly_poweroff cannot be called from atomic context.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>