[Nouveau] we don't synchronize in our buffer object move driver hook

[Maarten Maathuis]

ttm_bo_validate() calls ttm_bo_move_buffer() which calls
ttm_bo_handle_move_mem() which calls bdev->driver->move() which calls
nouveau_bo_move_m2mf(), which doesn't ensure synchronization (nowait
== false) if the bo is moved on another channel (which is common for
nv50). I suspect this to be the source of some corruption problems.

Maarten.