x86, AMD IOMMU: clean up Kconfig entry

AMD_IOMMU should depend on IOMMU_HELPER since they are the IOMMU
helper functions. SWIOTLB requires IOMMU_HELPER so declaring that
AMD_IOMMU depends on SWIOTLB properly fixes the problems.

Signed-off-by: Ingo Molnar <mingo@elte.hu>