drm/vc4: hdmi: Move clock calculation into its own function

The code to compute our clock rate for a given setup will be called in
multiple places in the next patches, so let's create a separate function
for it.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220222164042.403112-4-maxime@cerno.tech