drm/atomic-helpers: Remove outdated comment

We forgot to clean this up when adding connector refcounting.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483044517-5770-10-git-send-email-daniel.vetter@ffwll.ch