drm/nouveau/therm: let the vbios decide on the automatic fan management mode

This should fix automatic fan management on fermi cards who do not have
0x46 entries in the thermal table.

On my nve6, the blob sets the default linear range from 40°C to 100°C
but my nvcf's default values are 40°C to 85°C. Let's keep 85 as a default
for everyone.

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