drm/i915: Consolidate {bxt,cnl,icl}_init_cdclk

The BXT and CNL functions were already basically identical, whereas
ICL's function tried to do its own sanitization rather than calling
bxt_sanitize_cdclk.

This should actually fix a bug in our ICL initialization where it would
consider the /2 CD2X divider invalid and force an unnecessary
sanitization (we now have valid clock frequencies that use this
divider).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910154252.30503-9-matthew.d.roper@intel.com