drm/nouveau/therm: make sure the temperature settings are sane on nv84+

One of my nv92 has a calibrated internal sensor but it displays 0°C
as the default values use sw calibration values to force the temperature
to 0.

Since we cannot read the temperature from the adt7473 present on this board,
let's re-enable the internal reading!

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