drm/vc4: hdmi: Use a device-managed action for DDC

The reference to the DDC controller device needs to be put back when we're
done with it. Let's use a device-managed action to simplify the driver.

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