drm/ttm: remove padding from ttm_ref_object on 64bit builds

Re-order structure ttm_ref_object to remove 8 bytes of alignment padding
on 64 bit builds, so shrinking its size from 72 to 64 bytes allowing it
to fit into a smaller slab.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>