drm/connector: Clarify when drm_connector_unregister is needed

The current documentation for drm_connector_unregister() mentions that
it's needed for connectors that have been registered through
drm_dev_register().

However, this was a typo and was meant to be drm_connector_register(),
which only applies to connectors registered after drm_dev_register() has
been called.

In addition, it was also mentioning that connectors are unregistered
automatically when drm_dev_unregister() is called. This part is a bit
misleading, since it might make it appear that
drm_connector_unregister() applies either to all connectors, or none of
them.

After discussing it with Daniel, it appears that we always need to call
drm_connector_unregister() on connectors that have been registered with
drm_connector_register(), but only those.

drm_connector_init() already mentions that it only needs
drm_connector_cleanup(), so let's clarify the drm_connector_register()
and drm_connector_unregister() documentation to point at each other, and
remove the misleading part about drm_dev_unregister().

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