media: s5p_cec: use new cec_notifier_parse_hdmi_phandle helper

The S5P CEC driver increased the HDMI device refcount when
it shouldn't. Use the new helper function to ensure that that
doesn't happen and to simplify the driver code.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>