Align rdfx_get_rdf_model() prototype with entity API naming convention

Entity API uses the following prototype for entity_metadata_wrapper():

entity_metadata_wrapper($type, $data = NULL, array $info = array())

in other words it accepts any kind of data as $data. rdfx_get_rdf_model() requires a entity ID but that's not convenient in certain conditions, for example see #1237090: SPARQL Endpoint index not immediately sync'ed on node update. I propose to align rdfx_get_rdf_model() to accept $data as second parameter instead of $id.