drm/i915/mtl: update DP 2.0 vswing table for C20 phy

Update preset 15 setting to align the latest bspec value.

Bspec: 74104
Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Clint Taylor <Clinton.A.Taylor@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606104238.31953-1-shawn.c.lee@intel.com

drivers/gpu/drm/i915/display/intel_ddi_buf_trans.c

@@ -1072,7 +1072,7 @@ static const union intel_ddi_buf_trans_entry _mtl_c20_trans_uhbr[] = {
 	{ .snps = { 37, 4, 7 } },       /* preset 12 */
 	{ .snps = { 33, 4, 11 } },      /* preset 13 */
 	{ .snps = { 40, 8, 0 } },	/* preset 14 */
-	{ .snps = { 28, 2, 2 } },	/* preset 15 */
+	{ .snps = { 30, 2, 2 } },	/* preset 15 */
 };
 
 /* HDMI2.0 */