drivers/pci/intel-iommu.c: fix build with older gcc's
drivers/pci/intel-iommu.c: In function `__iommu_calculate_agaw':
drivers/pci/intel-iommu.c:437: sorry, unimplemented: inlining failed in call to 'width_to_agaw': function body not available
drivers/pci/intel-iommu.c:445: sorry, unimplemented: called from here
Move the offending function (and its siblings) to top-of-file, remove the
forward declaration.