drm/vc4: hdmi: Pass vc4_hdmi to CEC code

Our CEC code also retrieves the associated vc4_hdmi by setting the
vc4_dev pointer as its private data, and then dereferences its vc4_hdmi
pointer.

In order to eventually get rid of that pointer, we can simply pass the
vc4_hdmi pointer directly.

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: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/cb575cb9e13018bce131b8535e5b572dc1027877.1599120059.git-series.maxime@cerno.tech