drm/atomic-helper: Mention the new system/resume helpers the docs

They have pretty kerneldoc already, but better to link to that in
one of the overview sections.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-26-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>