gallium/util: Really allow aliasing of dst for u_box_union_*

gallium/util: Really allow aliasing of dst for u_box_union_*

Gallium nine relies on aliasing to work with this function.
Without this patch, dirty region tracking was incorrect, which
could lead to incorrect textures or vertex buffers.
Fixes several game bugs with nine.
Fixes https://github.com/iXit/Mesa-3D/issues/234

Signed-off-by: Axel Davy <axel.davy@ens.fr>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>