drm/i915: Reuse intel_adjusted_rate() for pfit pixel rate adjustment

Replace the hand rolled pfit downscale calculations with
intel_adjusted_rate().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210330184254.6290-2-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>