drm/dp: Add eDP 1.5 bit definition

Add the eDP revision bit value for 1.5.

Spec: eDPv1.5 Table 16-5
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Tested-by: Ben Kao <ben.kao@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250206063253.2827017-2-suraj.kandpal@intel.com

Author: surajk8

include/drm/display/drm_dp.h

@@ -996,6 +996,7 @@
 # define DP_EDP_14			    0x03
 # define DP_EDP_14a                         0x04    /* eDP 1.4a */
 # define DP_EDP_14b                         0x05    /* eDP 1.4b */
+# define DP_EDP_15			    0x06    /* eDP 1.5 */
 
 #define DP_EDP_GENERAL_CAP_1		    0x701
 # define DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP		(1 << 0)