drm/nouveau: Put the dithering check back in nouveau_connector_create.

a7b9f9e5adef dropped it by accident.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Tested-by: Thibaut Girka <thib@sitedethib.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>