drm/i915/cnl: Add missing type case.

Paulo had noticed that inside cnl_ddi_vswing_program
the case was handling voltage but with no indication
of type where a missing type could also take us to that
path. So my first attempt was to add a message to
let clear who trigger that path.

However DK had a better idea that is to handle the
missed type sooner before it might take to that path.
So this v2 here uses his approach.

v2: Handle missed type sooner.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170710205852.28352-1-rodrigo.vivi@intel.com