drm/vc4: hdmi: Switch to device-managed CEC initialization

The current code to unregister our CEC device needs to be undone manually
when we remove the HDMI driver.

Since the CEC framework will allocate its main structure, and will defer
its deallocation to when the last user will have closed it, we don't really
need to take any particular measure to prevent any use-after-free and can
thus use any managed action.

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