x86, AMD IOMMU: don't try to init IOMMU if early detect code did not detect one

This patch adds a check if the early detect code has found AMD IOMMU hardware
descriptions and does not try to initialize hardware if the check failed.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: robert.richter@amd.com
Cc: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>