drm/vc4: hdmi: Move CEC init to its own function

The CEC init code was put directly into the bind function, which was quite
inconsistent with how the audio support was done, and would prevent us from
further changes to skip that initialisation entirely.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/21f4717e076291522d0784a7fd3774d8e97eaf01.1599120059.git-series.maxime@cerno.tech