drm/nouveau/disp: replace hda func pointer check with flag

Simpler, and less error-prone than a separate set of function pointers.

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