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>

drivers/gpu/drm/i915/intel_bios.c

@@ -672,8 +672,13 @@ parse_edp(struct drm_i915_private *dev_priv, struct bdb_header *bdb)
 	if (bdb->version >= 173) {
 		uint8_t vswing;
 
-		vswing = (edp->edp_vswing_preemph >> (panel_type * 4)) & 0xF;
-		dev_priv->vbt.edp_low_vswing = vswing == 0;
+		/* Don't read from VBT if module parameter has valid value*/
+		if (i915.edp_vswing) {
+			dev_priv->edp_low_vswing = i915.edp_vswing == 1;
+		} else {
+			vswing = (edp->edp_vswing_preemph >> (panel_type * 4)) & 0xF;
+			dev_priv->edp_low_vswing = vswing == 0;
+		}
 	}
 }