drm/dp: Add DP_DPCD_REV_XX to drm_dp_helper

As more differentation occurs between DP spec. Its useful to have these
as macros in a drm_dp_helper.

v2: DPCD_REV_XX to DP_DPCD_REV_XX
Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180504221800.17830-1-matthew.s.atwood@intel.com

include/drm/drm_dp_helper.h

@@ -64,6 +64,11 @@
 /* AUX CH addresses */
 /* DPCD */
 #define DP_DPCD_REV                         0x000
+# define DP_DPCD_REV_10                     0x10
+# define DP_DPCD_REV_11                     0x11
+# define DP_DPCD_REV_12                     0x12
+# define DP_DPCD_REV_13                     0x13
+# define DP_DPCD_REV_14                     0x14
 
 #define DP_MAX_LINK_RATE                    0x001