drm/vc4: hdmi: Enable the audio clock

The audio clock is used by the HDMI controller driver and we were using
it to get its audio rate and compute the dividers needed to reach a
given audio sample rate.

However, we were never enabling it, which was resulting in lockups on
the BCM2712.

Fixes: 632ee3aa ("drm/vc4: hdmi: Add audio-related callbacks")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>