drm/ttm: fix handling of TTM_PL_FLAG_TOPDOWN v2

bo->mem.placement is not initialized when ttm_bo_man_get_node is called,
so the flag had no effect at all.

v2: change nouveau and vmwgfx as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>