drm/nouveau/therm: implement support for temperature alarms

For now, we only boost the fan speed to the maximum and auto-mode
when hitting the FAN_BOOST threshold and halt the computer when it
reaches the shutdown temperature. The downclock and critical thresholds
do nothing.

On nv43:50 and nva3+, temperature is polled because of the limited hardware.
I'll improve the nva3+ situation by implementing alarm management in PDAEMON
whenever I can but polling once every second shouldn't be such a problem.

v2 (Ben Skeggs):
- rebased

v3: fixed false-detections and threshold reprogrammation handling on nv50:nvc0

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