x86 gart: don't complain if no AMD GART found

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: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>