drm/nouveau/fan: add toggle fan support

v2: change percent from int to atomic_t
v3: random fixes
v4 (Ben Skeggs):
- adapted for split-out fan-control "protocol" structure
- removed need for timer resched
- support for forcing 'toggle' control on PWM boards

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