AMD IOMMU: use iommu_device_max_index

AMD IOMMU can use iommu_device_max_index() instead of the homegrown
function.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>