drm/nouveau/drm/therm/fan: add a fallback if no fan control is specified in the vbios

This seems to be absolutely necessary for a lot of NV40.

Reported-by: gsgf on IRC/freenode
Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>