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

drivers/gpu/drm/drm_dp_helper.c

@@ -1158,6 +1158,7 @@ EXPORT_SYMBOL(drm_dp_psr_setup_time);
 /**
  * drm_dp_start_crc() - start capture of frame CRCs
  * @aux: DisplayPort AUX channel
+ * @crtc: CRTC displaying the frames whose CRCs are to be captured
  *
  * Returns 0 on success or a negative error code on failure.
  */