drm/nouveau/disp: collapse nvkm_dp into nvkm_outp

There should be no changes to code here other than modifying the
dereferences.

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