IA64: fix compile error on IA64_DIG_VTD

This moves iommu_detected to arch/ia64/kernel/dma-mapping.c from
arch/ia64/kernel/pci-swiotlb.c to fix the following error on on
IA64_DIG_VTD:

 arch/ia64/kernel/built-in.o: In function `pci_iommu_init':
 pci-dma.c:(.init.text+0xa021): undefined reference to `iommu_detected'
 pci-dma.c:(.init.text+0xa030): undefined reference to `iommu_detected'
 drivers/built-in.o: In function `detect_intel_iommu':
 (.init.text+0x11c0): undefined reference to `iommu_detected'
 drivers/built-in.o: In function `detect_intel_iommu':
 (.init.text+0x11e1): undefined reference to `iommu_detected'

iommu_detected is used to handle IOMMUs so I guess that
arch/ia64/kernel/dma-mapping.c is ok (there might be a better place
for it though).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>