drm: Add EDID helper documentation

Add a reference section about the EDID helper functions to the DRM
documentation.
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>

Documentation/DocBook/drm.tmpl

@@ -2151,6 +2151,10 @@ void intel_crt_init(struct drm_device *dev)
 !Iinclude/drm/drm_dp_helper.h
 !Edrivers/gpu/drm/drm_dp_helper.c
     </sect2>
+    <sect2>
+      <title>EDID Helper Functions Reference</title>
+!Edrivers/gpu/drm/drm_edid.c
+    </sect2>
   </sect1>
 
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