drm/atomic-helper: use for_each_*_in_state more

This avois leaking drm_atomic_state internals into the helpers. The
only place where this still happens after this patch is drm_atomic_helper_swap_state().
It's unavoidable there, and maybe a good indicator we should actually
move that function into drm_atomic.c.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-2-git-send-email-daniel.vetter@ffwll.ch