Existing core code/drivers call drm_mm_put_block on ttm_mem_reg.mm_node
directly.  Future patches will modify TTM behaviour in such a way that
ttm_mem_reg.mm_node doesn't necessarily belong to drm_mm.
Reviewed-by: NJerome Glisse <jglisse@redhat.com>
Acked-by: NThomas Hellström <thellstrom@vmware.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>