drm/i915: move edp low vswing config to vbt data

Move all data initialized from VBT under dev_priv->vbt. No functional
changes.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458834623-8734-3-git-send-email-jani.nikula@intel.com