drm/nouveau/pm: introduce generic handler for on-chip fan controller

The handling of the internal pwm fan controller is similar enough between
current chipsets that it makes sense to share the logic, and bugfixes :)

No hw backends converted yet, will automatically fall-through to the
"old" per-chipset fanspeed hooks for now.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>