drm/nouveau: Move the fence wait before migration resource clean-up.

Avoids an oops in the fence wait failure path (bug 26521).

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Tested-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>