drm/bridge/synopsys: dw-hdmi: Add an option to suppress loading CEC driver

This adds DW-HDMI driver a glue option to disable loading of the CEC sub-driver.

On some SoCs, the CEC functionality is enabled in the IP config bits, but the
CEC bus is non-functional like on Amlogic SoCs, where the CEC config bit is set
but the DW-HDMI CEC signal is not connected to a physical pin, leading to some
confusion when the DW-HDMI CEC controller can't communicate on the bus.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416092737.1971876-2-narmstrong@baylibre.com