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>

drivers/gpu/drm/i915/i915_drv.h

@@ -1466,6 +1466,7 @@ struct intel_vbt_data {
 	bool edp_initialized;
 	bool edp_support;
 	int edp_bpp;
+	bool edp_low_vswing;
 	struct edp_power_seq edp_pps;
 
 	struct {

drivers/gpu/drm/i915/intel_bios.c

@@ -662,6 +662,13 @@ parse_edp(struct drm_i915_private *dev_priv, struct bdb_header *bdb)
 			      edp_link_params->vswing);
 		break;
 	}
+
+	if (bdb->version >= 173) {
+		uint8_t vswing;
+
+		vswing = (edp->edp_vswing_preemph >> (panel_type * 4)) & 0xF;
+		dev_priv->vbt.edp_low_vswing = vswing == 0;
+	}
 }
 
 static void

drivers/gpu/drm/i915/intel_bios.h

@@ -554,6 +554,7 @@ struct bdb_edp {
 	/* ith bit indicates enabled/disabled for (i+1)th panel */
 	u16 edp_s3d_feature;
 	u16 edp_t3_optimization;
+	u64 edp_vswing_preemph;		/* v173 */
 } __packed;
 
 struct psr_table {