drm/doc: diagram for mode objects and properties

Resulted in confusion a few times in the past.

v2: Spelling fix (Eric).

Cc: Eric Anholt <eric@anholt.net>
Acked-by: Eric Anholt <eric@anholt.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302151638.1882-5-daniel.vetter@ffwll.ch