drm/ttm: keep a reference to transfer pipelined BOs

Make sure the transfered BO is never destroy before the transfer BO.

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