drm/doc: Add KMS overview graphs

Oh, the shiny and pretties!

v2: Review from Laurent.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302151638.1882-3-daniel.vetter@ffwll.ch