drm/ttm: Don't unreserve swapped BOs that were previously reserved

If ttm_bo_swapout doesn't own the lock, don't release it. Someone
else probably depends on it still being locked.

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