drm/ttm: fix adding foreign BOs to the LRU during init v2

If we import a BO with an external reservation object we don't
reserve/unreserve it. So we never add it to the LRU causing a possible
denial of service.

v2: fix typo in commit message

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>