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: 9ce523cc ("drm/nouveau: separate buffer object backing memory from nvkm structures")
Signed-off-by: Danilo Krummrich <dakr@kernel.org>