drm/dp: Add missing description to parameter

Gabriel Krisman reported these warnings when building the documentation:

 ./drivers/gpu/drm/drm_dp_helper.c:1165: warning: No description found
for parameter 'crtc'
./drivers/gpu/drm/drm_dp_helper.c:1166: warning: No description found
for parameter 'crtc'

Reported-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307203511.14258-1-tomeu.vizoso@collabora.com