make htmldocs helps with catching these.

Cc: Dave Airlie <airlied@gmail.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170718074113.5554-1-daniel.vetter@ffwll.chSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>