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

The uninitialize flow is the same on all of these platforms, aside from
calculating a different frequency level.

v2: Reverse platform conditional order for consistency.  (Ville)

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/20190910154252.30503-6-matthew.d.roper@intel.com