drm/nouveau/ttm: wait for bo fence to signal before unmapping vmas

TTM was changed a while back to allow for pipelining of buffer moves, and
part of this was the removal of waiting for a BO to idle before calling
move(), placing the responsibility on the driver to do this if required.

That's all well and good, except, we make use of move_notify() to handle
mapping/unmapping from the GPU VMM as move() isn't called on all paths.

This commit adds a wait before unmapping from a VMM in move_notify(), to
prevent GPU page faults where a buffer is still being accessed.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org [v4.8+]

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -1209,6 +1209,7 @@ nouveau_bo_move_ntfy(struct ttm_buffer_object *bo, struct ttm_mem_reg *new_mem)
 			       nvbo->page_shift != vma->vm->mmu->lpg_shift)) {
 			nvkm_vm_map(vma, new_mem->mm_node);
 		} else {
+			WARN_ON(ttm_bo_wait(bo, false, false));
 			nvkm_vm_unmap(vma);
 		}
 	}