drm/vc4: hdmi: Switch to DRM-managed connector initialization

The current code will call drm_connector_unregister() and
drm_connector_cleanup() when the device is unbound. However, by then, there
might still be some references held to that connector, including by the
userspace that might still have the DRM device open.

Let's switch to a DRM-managed initialization to clean up after ourselves
only once the DRM device has been last closed.

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