drm/doc: atomic overview, with graph

I want to split up a few more things and document some details better
(like how exactly to subclass drm_atomic_state). And maybe also split
up the helpers a bit per-topic, but this should be a ok-ish start for
better atomic overview.

v2: Spelling and clarifications (Eric).

v3: Implement suggestion from Gabriel to fix the graph.

v4: Review from Laurent.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Acked-by: Eric Anholt <eric@anholt.net>
Cc: Eric Anholt <eric@anholt.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302151638.1882-6-daniel.vetter@ffwll.ch