drm/ttm: fix ttm_mem_evict_first once more

The code path isn't hit at the moment, but we need to take the lock to
add the BO back to the LRU.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>