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

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

BugLink: http://bugs.launchpad.net/bugs/1546572
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>
(cherry picked from commit ed704a43e84cc536081423dcd3491acf2791aaeb)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>