drm/i915: Kill cnl_sanitize_cdclk()

The CNL variant of this function is identical to the BXT variant aside
from not needing to handle SSA precharge.

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/20190910154252.30503-5-matthew.d.roper@intel.com