Impact: remove annoying bootup printk

It's perfectly normal for no AMD GART to be present, e.g., if you have
Intel CPUs.  None of the other iommu_init() functions makes noise when
it finds nothing.
Signed-off-by: NBjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: NJoerg Roedel <joerg.roedel@amd.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>