drm/i915: Reuse cnl_modeset_calc_cdclk() on icl+

The cnl and icl .modeset_calc_cdclk() functions are identical. Drop one
copy.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911133129.27466-3-ville.syrjala@linux.intel.com