drm: sti: use cec_notifier_conn_(un)register

Use the new cec_notifier_conn_(un)register() functions to
(un)register the notifier for the HDMI connector, and fill
in the cec_connector_info.

Changes since v2:
	Don't invalidate physical address before unregistering the
	notifier.

Signed-off-by: Dariusz Marcinkiewicz <darekm@google.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190814104520.6001-7-darekm@google.com