drm/i915/bxt: Rename broxton to bxt in PHY/CDCLK function prefixes

Rename these remaining function prefixes to better align with the
corresponding SKL functions.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>