drm/nv40/therm: reset temperature sensor on init

Current uninitialized sensor detection does not work for me on nv4b and
sensor returns crazy values (>190°C). It stabilises later, but it's too
late - therm code shutdowns the machine...

Let's just reset it on init.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>