drm/ttm: Do not add non-system domain BO into swap list

BO would be added into swap list if it is validated into system domain.
If BO is validated again into non-system domain, say, VRAM domain. It
actually should not be in the swap list.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>