drm/nouveau/disp: add output detect method

This will check the relevant hotplug pin and skip the DDC probe we
currently do if a display is present.

- preparation for GSP-RM.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Danilo Krummrich <me@dakr.org>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230919220442.202488-8-lyude@redhat.com