drm/doc: Add KMS overview graphs

Oh, the shiny and pretties!

v2: Review from Laurent.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302151638.1882-3-daniel.vetter@ffwll.ch
diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 03040aa..012b6ff 100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -114,6 +114,8 @@
 .. kernel-doc:: drivers/gpu/drm/drm_fb_cma_helper.c
    :export:
 
+.. _drm_bridges:
+
 Bridges
 =======
 
@@ -139,6 +141,8 @@
 .. kernel-doc:: drivers/gpu/drm/drm_bridge.c
    :export:
 
+.. _drm_panel_helper:
+
 Panel Helper Reference
 ======================