drm/doc: Use new substruct support

Support for this just recently landed in linux-next.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180219225356.24996-5-daniel.vetter@ffwll.ch