drm/nouveau/therm: add support for fan-control modes

For now, only 2 control modes are available:
- NONE: The fan is never touched (default)
- MANUAL: The fan is set to the user-defined fan speed (pwm1)

This patch introduces a distinction between ptherm internal fan management
and external fan management. The latter is bound to respect the fan mode
while the first can still select the speed it wants unless the NONE mode
is selected. This is important for automatic fan management.

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