drm/nouveau/pbus: add a PBUS subdev that hands IRQs to the right subdevs

We are going to use PTHERM's IRQs for thermal monitoring but we need to route
them first.

On nv31-50, PBUS's IRQ line is shared with GPIOs IRQs.

It seems like nv10-31 GPIO interruptions aren't well handled. I kept the
original behaviour but it is wrong and may lead to an IRQ storm.

Since we enable all PBUS IRQs, we need a way to avoid being stormed if we
don't handle them. The solution I used was to mask the IRQs that have not been
handled. This will also print one message in the logs to let us know.

v2: drop the shared intr handler because of was racy
v3: style fixes
v4: drop a useless construct in the chipset-dependent INTR
v5: add BUS to the disable mask
v6 (Ben Skeggs):
- general tidy to match the rest of the driver's style
- nva3->nvc0, nva3 can be serviced just fine with nv50.c, rnndb even notes
  that the THERM_ALARM bit got left in the hw until fermi anyway.. so, it's
  not going to conflict
- removed the peephole and user stuff, for the moment.. will handle them
  later if we find a good reason to actually care..
- limited INTR_EN to just what we can handle for now, mostly to prevent
  spam of unknown status bits (seen on at least nv4x)

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