drm/nouveau/disp: add connector class

Will be used to provide more solid driver interfaces in general, but
the immediate motivation is work towards fixing issues with handling
hotplug/DP IRQ events.

Its use is currently limited to where we support non-polled hotplug
already (ie. any GPU since NV40ish era, where our DCB handling works
well enough), until that gets cleaned up someday.

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>