drm/connector: Mention the cleanup after drm_connector_init

Unlike encoders and CRTCs, the drm_connector_init() and
drm_connector_init_with_ddc() don't mention how the cleanup is supposed to
be done. Let's add it.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>