Documentation/gpu: Fix no structured comments warning for drm_gem_ttm_helper.h

Fixes
include/drm/drm_gem_ttm_helper.h:1: warning: no structured comments found

Fixes: ff540b76 ("drm/ttm: add drm gem ttm helpers, starting with drm_gem_ttm_print_info()")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920193558.89815-2-sean@poorly.run

Documentation/gpu/drm-mm.rst

@@ -406,9 +406,6 @@ GEM TTM Helper Functions Reference
 .. kernel-doc:: drivers/gpu/drm/drm_gem_ttm_helper.c
    :doc: overview
 
-.. kernel-doc:: include/drm/drm_gem_ttm_helper.h
-   :internal:
-
 .. kernel-doc:: drivers/gpu/drm/drm_gem_ttm_helper.c
    :export: