drm/vc4: hdmi: Add CEC support flag

Similarly to the audio support, CEC support is not there yet for the
BCM2711, so let's skip entirely the CEC initialization through a variant
flag.

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/bd0c4afa83b4e121692352cdc2dd1886162c7552.1599120059.git-series.maxime@cerno.tech