drm/i915: Implement connector state duplication

So that we can add connector states to the drm_atomic_state used in the
legacy modeset.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_crt.c

@@ -794,6 +794,7 @@ static const struct drm_connector_funcs intel_crt_connector_funcs = {
 	.destroy = intel_crt_destroy,
 	.set_property = intel_crt_set_property,
 	.atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
+	.atomic_duplicate_state = drm_atomic_helper_connector_duplicate_state,
 	.atomic_get_property = intel_connector_atomic_get_property,
 };