nv04-nv40: fix nv##_surface_copy() for flipped

nv04-nv40: fix nv##_surface_copy() for flipped

If do_flipp is true, it would first do the proper copy, height would
wrap around to unsigned maximum, and then it attempts to do another
copy.

Return after doing the proper copy.

Signed-off-by: Pekka Paalanen <pq@iki.fi>