drm/ttm: Fix a potential comparison of structs.

On some architectures the comparison may cause a compilation failure.

Original partial fix Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Dave Airlie <airlied@redhat.com>