drm: Fix DisplayPort branch device ID kernel-doc

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: 266d783baaf5 ("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