x86: Move iommu_shutdown_noop to x86_init.c

iommu_init_noop() is in arch/x86/kernel/x86_init.c but
iommu_shutdown_noop() in arch/x86/include/asm/iommu.h.

This moves iommu_shutdown_noop() to x86_init.c for consistency.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
LKML-Reference: <1258199198-16657-3-git-send-email-fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>