drm/nouveau: fix bad ret code in nouveau_bo_move_prep

In `nouveau_bo_move_prep`, if `nouveau_mem_map` fails, an error code
should be returned. Currently, it returns zero even if vmm addr is not
correctly mapped.

Cc: stable@vger.kernel.org
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Shuhao Fu <sfual@cse.ust.hk>
Fixes: 9ce523c ("drm/nouveau: separate buffer object backing memory from nvkm structures")
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: sfu2

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -929,7 +929,7 @@ nouveau_bo_move_prep(struct nouveau_drm *drm, struct ttm_buffer_object *bo,
 		nvif_vmm_put(vmm, &old_mem->vma[1]);
 		nvif_vmm_put(vmm, &old_mem->vma[0]);
 	}
-	return 0;
+	return ret;
 }
 
 static int