drm/ttm: fix adding foreign BOs to the swap LRU

It doesn't make any sense to try to swap out imported BOs.

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>