drm/vc4: hdmi: Move clock validation to its own function

Our code is doing the same clock rate validation in multiple instances.
Let's create a helper to share the rate validation.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220222164042.403112-3-maxime@cerno.tech