drm/connector: Introduce drmm_connector_init

Unlike other DRM entities, there's no helper to create a DRM-managed
initialisation of a connector.

Let's create an helper to initialise a connector that would be passed as an
argument, and handle the cleanup through a DRM-managed action.

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