drm/nouveau/disp: modify OR allocation policy to account for HDA requirements

Since GM200, SORs are no longer tied to a specific connector, and we
allocate them instead, with the assumption that all SORs are equally
capable.

However, there's a 1<->1 mapping between SOR and HDA pin widget, and
it turns out that it's possible for some widgets to be disabled...

In order to avoid picking a SOR without a valid pin widget, some new
rules need to be added.

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