drm/nouveau/kms/nv50-: Reverse args for nv50_outp_get_(old|new)_connector()

Just to be more consistent with the order of args that DRM helpers like
drm_atomic_get_new_crtc_state() use.

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