drm/atomic: doc: remove old comment about nonblocking commits

We now support nonblocking commits on drm_atomic_helper_commit()
so the comment is not valid anymore.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1480946626-30917-1-git-send-email-gustavo@padovan.org