target-microblaze: Consolidate MMU enabled checks

Consolidate MMU enabled checks to cpu_mmu_index().
No functional changes.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>