drm/i915/skl: Add module parameter to select edp vswing table

This provides an option to override the value set by VBT
for selecting edp Vswing Pre-emph setting table.

v2: Adding comment about this being a temporary workaround and
making the parameter read-only (Jani)
v3: Changing mode to 0400 instead of 0 (Jani)

https://bugs.freedesktop.org/show_bug.cgi?id=89554
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_ddi.c b/drivers/gpu/drm/i915/intel_ddi.c
index d5bee8b..b8eabc6 100644
--- a/drivers/gpu/drm/i915/intel_ddi.c
+++ b/drivers/gpu/drm/i915/intel_ddi.c
@@ -282,7 +282,7 @@
 		ddi_translations_fdi = NULL;
 		ddi_translations_dp = skl_ddi_translations_dp;
 		n_dp_entries = ARRAY_SIZE(skl_ddi_translations_dp);
-		if (dev_priv->vbt.edp_low_vswing) {
+		if (dev_priv->edp_low_vswing) {
 			ddi_translations_edp = skl_ddi_translations_edp;
 			n_edp_entries = ARRAY_SIZE(skl_ddi_translations_edp);
 		} else {