drm: remove legacy drm_connector_property fxns

Replace references to and remove the connector property fxns, which
have been superseded with the more general object property fxns:

  + drm_connector_attach_property -> drm_object_attach_property
  + drm_connector_property_set_value -> drm_object_property_set_value
  + drm_connector_property_get_value -> drm_object_property_get_value

Signed-off-by: Rob Clark <rob@ti.com>