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>

arch/x86/Kconfig

@@ -575,7 +575,7 @@ config SWIOTLB
 	  3 GB of memory. If unsure, say Y.
 
 config IOMMU_HELPER
-	def_bool (CALGARY_IOMMU || GART_IOMMU || SWIOTLB)
+	def_bool (CALGARY_IOMMU || GART_IOMMU || SWIOTLB || AMD_IOMMU)
 config MAXSMP
 	bool "Configure Maximum number of SMP Processors and NUMA Nodes"
 	depends on X86_64 && SMP