target/s390x: Use cpu_*_mmu instead of helper_*_mmu

The helper_*_mmu functions were the only thing available
when this code was written.  This could have been adjusted
when we added cpu_*_mmuidx_ra, but now we can most easily
use the newest set of interfaces.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>