drm/nouveau/pm: manual pwm fanspeed management for nv40+ boards

Exposes the following sysfs entries:
- fan0_input: read the rotational speed of the fan (poll a bit during 250ms)
- pwm0: set the pwm duty cycle
- pwm0_min/max: set the minimum/maximum pwm value

v2 (Ben Skeggs):
- nv50 pwm controller code removed in favour of other more complete code
- FAN_RPM -> FAN_SENSE
- merged FAN_SENSE readout into common code, not at all nv50-specific
- protected fanspeed changes with perflvl_wr
- formatting tidying
- added some comments where things are shaky

v3 (Martin Peres)
- ensure duty min/max from thermal table are sane

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