drm: update todo.rst

Just drive-by, but we have gsoc running so better to update it now.

Great news is that two entries can be removed because essentially all
done.

v2: Keep a bunch of the todos, Gabriel is working on them.

Cc: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170404095304.17599-1-daniel.vetter@ffwll.ch