Make connector registration automatic

[Chris Wilson]

The current recommendation is for new drivers to call:
        drm_dev_register();
        drm_connector_register_all();
because of a limitation in the API for old drivers using driver->load()
and being unable to call drm_connector_register() multiple times. Now
that drm_connector_register() is safe for multiple uses, we can combine
the two steps into one (drm_dev_register()).
-Chris