Update function writes double-serialized values to the database

The update function introduced in #2354437: New revisions should always clone a custom display uses drupal_write_record. In update functions the DB schema might not be loaded, so db_update should be used instead.

It gave me problems with serialized values in the panelizer_entity table in the fields contexts and relationships, so panelized entities with either set went down in flames because the values in the fields could not be de-serialized.

The patch attached changes the update function to use db_update.