drm/nouveau/disp: add common class handling between <nv50 and >=nv50

About to expose head/output path/connector objects everywhere, so we will
need support for child classes prior to nv50 now.

Somewhat cleaner than the code >=nv50 used previously.

v2:
- use ?: (lyude)

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