drm/vc4: hdmi: Remove unused CEC_CLOCK_DIV define

The CEC_CLOCK_DIV define is not used anywhere in the driver, let's remove
it.

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/53805cc47e99a137e5f08c92fb781cc2e2134299.1599120059.git-series.maxime@cerno.tech