drm/nouveau/disp: add output class

Will be used to more cleanly implement existing method interfaces that
take some confusing (IEDTkey, inherited from VBIOS, which RM no longer
uses on Ampere) match values to determine which display path to operate
on.

Methods will be protected from racing with supervisor, and from being
called where they shouldn't be (ie. without an OR assigned).

v2:
- use ?: (lyude)
v3:
- fix return code if noacquire() method fails

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>