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