drm/ttm: fix ttm_bo_unreserve

Since we now keep BOs on the LRU we need to explicitely remove
them from the LRU now after they are pinned.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>