drm: Fix DisplayPort branch device ID kernel-doc

Fix missing parameter description for DisplayPort branch device ID.
This fixes warning of "No description found for parameter 'id[6]'" when
creating documentation by 'make htmldocs'.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
References: https://lists.freedesktop.org/archives/intel-gfx/2016-September/106645.html
Fixes: 266d783 ("drm: Read DP branch device id")
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474022355-29990-1-git-send-email-mika.kahola@intel.com

drivers/gpu/drm/drm_dp_helper.c

@@ -516,6 +516,7 @@ EXPORT_SYMBOL(drm_dp_downstream_max_bpc);
 /**
  * drm_dp_downstream_id() - identify branch device
  * @aux: DisplayPort AUX channel
+ * @id: DisplayPort branch device id
  *
  * Returns branch device id on success or NULL on failure
  */