Make the BDW and SKL code a bit more similar by extracting the
low vswing handling for BDW into a helper, as we already have
it like that for SKL+.

Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-10-git-send-email-ville.syrjala@linux.intel.comReviewed-by: NImre Deak <imre.deak@intel.com>