drm/nouveau/therm: turn on fan only when threshold hit in positive direction

+ the same for shutdown threshold - seems impossible, but shutdown can fail.

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>