drm/i915/bxt: eDP low vswing support

Adding voltage swing table for edp to support low vswings.

v2: Rebased.

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>