drm/nouveau: Fix merge commit

The most recent merge commit in airlied/drm-next has problems with
confusing old_crtc_state and new_crtc_state. Use the
for_each_oldnew_crtc_in_state macros to clean up the confusion,
and explicitly look at the correct state instead of looking at
asyh->state.

With these fixes it becomes more obvious what the code is trying to do,
which will hopefully prevent future confusion.

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>