drm/nouveau/pm: fix a typo related to the move to the therm subdev

Reported-by: Vekin on IRC
Reported-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>