drm/nouveau/therm: ack any pending IRQ at init

This is safe because ptherm hasn't been configured yet and will be a
little further down the initialization path. Ptherm should be safe
regarding to runtime reconfiguration.

v2:
  - do not limit this patch to nv84-a3 and make it nv84+

v3:
  - move the ack to fini()
  - disable IRQs on fini()
  - silently ignore un-requested IRQs

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