drm/nouveau: Convert nouveau to use new iterator macros, v2.

Use the new atomic iterator macros, the old ones are about to be
removed. With the new macros, it's more easy to get old and new state so
get them from the macros instead of from obj->state.

Changes since v1:
- Don't mix up old and new state. (danvet)
- Rebase on top of interruptible swap_state changes.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20170719143920.25685-7-maarten.lankhorst@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>