drm/meson: dw-hdmi: disable DW-HDMI CEC sub-driver

On the Amlogic SoCs, the DW-HDMI HW support is here but the DW-HDMI CEC signal
is not connected to a physical pin, leading to confusion when the dw-hdmi cec
controller can't communicate on the bus.

Disable it to avoid exposing a non-functinal bus.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416092737.1971876-3-narmstrong@baylibre.com