drm/connector: Consolidate Connector Initialization

We're going to add a DRM-managed connector initialization function.
Since we'll need both the with and without the DDC pointer, having a
single function that takes an optional pointer is easier to maintain.

Let's create a static function that will back both existing variants,
and will be reused by the DRM-managed variant.

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