Add support for cec-notifier to the cec-gpio driver.

This makes it possible to associate the CEC gpio pin with an HDMI
connector. This feature was always documented in the cec-gpio bindings:

Documentation/devicetree/bindings/media/cec-gpio.txt

But support for the hdmi-phandle property was never actually implemented in
this driver.

This patch adds support for this property.

It also fixes a few incorrect error returns in the probe() function, which
skipped the call to cec_delete_adapter().

Tested on a Raspberry Pi 3B with a modified vc4 driver.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>