drm/doc: Drop empty include for drm_color_mgmt.h

I'm fed up staring at the error message from kernel-doc that
it can't find anything.
Acked-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-10-daniel.vetter@ffwll.ch

Documentation/gpu/drm-kms.rst

@@ -523,9 +523,6 @@ Color Management Properties
 .. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
    :doc: overview
 
-.. kernel-doc:: include/drm/drm_color_mgmt.h
-   :internal:
-
 .. kernel-doc:: drivers/gpu/drm/drm_color_mgmt.c
    :export: