drm/nouveau/disp: introduce input/output resource abstraction

In order to properly support the SOR -> SOR + pad macro separation
that occurred with GM20x GPUs, we need to separate OR handling out
of the output path code.

This will be used as the base to support ORs (DAC, SOR, PIOR).

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