drm/i915: Extract intel_adjusted_rate()

Extract a small helper to calculate the downscaling
adjusted pixel rate/data rate/etc.

v2: Drop the plane visibility check and add a comment explaining why

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210401154043.19466-1-ville.syrjala@linux.intel.com