drm/i915/cnl: Move voltage check into ddi buf trans functions.

Let's start converging CNL buf translations to same style
used on previous platforms. So first thing is to use the
standard signature so we don't need to propagate the voltage
check into other parts of the code, but only on the parts
that it is really useful.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829232230.23051-5-rodrigo.vivi@intel.com