amd-iommu: don't free dma adresses below 512MB with CONFIG_IOMMU_STRESS

This will test the automatic aperture enlargement code. This is
important because only very few devices will ever trigger this code
path. So force it under CONFIG_IOMMU_STRESS.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>