drm/i915: Combine bxt_set_cdclk and cnl_set_cdclk

We'd previously combined ICL/TGL logic into the cnl_set_cdclk function,
but BXT is pretty similar as well.  Roll the cnl/icl/tgl logic back into
the bxt function; the only things we really need to handle separately
are punit notification and calling different functions to enable/disable
the cdclk PLL.

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/20190910154252.30503-4-matthew.d.roper@intel.com