x86, AMD IOMMU: honor iommu=off instead of amd_iommu=off

This patch removes the amd_iommu=off kernel parameter and honors the generic
iommu=off parameter for the same purpose.

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>