drm/nouveau/fan: handle the cases where we are outside of the linear zone

This fixes a bug where, when temperature is outside of the linear range, fan
pwm would be outside of the allowed range ([0, 100]) and could get negative in
some cases.

It seems like a regression that happened when we re-worked the fan management
logic before merging.

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