drm/ttm: make sure that we always zero init mem.bus v2

We are trying to remove the io_lru handling and depend
on zero init base, offset and addr here.

v2: init addr as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/388642/