drm: Read DP branch device id

Read DisplayPort branch device id string.

Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473419458-17080-6-git-send-email-mika.kahola@intel.com
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index f3d1424..faea76b 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -445,6 +445,7 @@
 #define DP_SOURCE_OUI			    0x300
 #define DP_SINK_OUI			    0x400
 #define DP_BRANCH_OUI			    0x500
+#define DP_BRANCH_ID                        0x503
 
 #define DP_SET_POWER                        0x600
 # define DP_SET_POWER_D0                    0x1
@@ -819,6 +820,7 @@
 				const u8 port_cap[4]);
 int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 			      const u8 port_cap[4]);
+int drm_dp_downstream_id(struct drm_dp_aux *aux, char id[6]);
 
 void drm_dp_aux_init(struct drm_dp_aux *aux);
 int drm_dp_aux_register(struct drm_dp_aux *aux);