drm/vc4: hdmi: Remove clock rate initialization

Now that the clock driver makes sure we never end up with a rate of 0,
the HDMI driver doesn't need to care anymore.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220225143534.405820-13-maxime@cerno.tech