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: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-10-daniel.vetter@ffwll.ch