drm/i915/skl: Support for edp low_vswing param in vbt

v2: Adding VBT version check for low_vswing field, and correcting parsing

v3: (Damien)
 - Restrain the scope of the 'vswing' variable
 - Use the more idiomatic "ev_priv->vbt.edp_low_vswing = vswing == 0;"
  instead of if (foo) var = true; else var = false;
 - Shorten edp_vswing_premph_setting to edp_vswing_premph to fit in 80 chars
 - Add the version from which the edp_vswing_premph field is valid in the
  struct definition

Reviewed-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v2)
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>