drm/vc4: hdmi: Split the CEC disable / enable functions in two

In order to ease further additions to the CEC enable and disable, let's
split the function into two functions, one to enable and the other to
disable.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210819135931.895976-5-maxime@cerno.tech