drm/vc4: hdmi: Don't register the CEC adapter if there's no interrupts

We introduced the BCM2711 support to the vc4 HDMI controller with 5.10,
but this was lacking any of the interrupts of the CEC controller so we
have to deal with the backward compatibility.

Do so by simply ignoring the CEC setup if the DT doesn't have the
interrupts property.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Tested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111142309.193441-13-maxime@cerno.tech