drm-kms-helpers.rst: document the DP CEC helpers

Document the Display Port CEC helper functions.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180711132909.25409-3-hverkuil@xs4all.nl

Documentation/gpu/drm-kms-helpers.rst

@@ -169,6 +169,15 @@ Display Port Helper Functions Reference
 .. kernel-doc:: drivers/gpu/drm/drm_dp_helper.c
    :export:
 
+Display Port CEC Helper Functions Reference
+===========================================
+
+.. kernel-doc:: drivers/gpu/drm/drm_dp_cec.c
+   :doc: dp cec helpers
+
+.. kernel-doc:: drivers/gpu/drm/drm_dp_cec.c
+   :export:
+
 Display Port Dual Mode Adaptor Helper Functions Reference
 =========================================================