drm/nouveau/therm: remove some confusion introduced by therm_mode

The kernel message "[  PTHERM][0000:01:00.0] Thermal management: disabled"
is misleading as it actually means "fan management: disabled".

This patch fixes both the source and the message to improve readability.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>