x86, AMD IOMMU: ignore detection of GART IOMMU

One of the last IOMMU updates covered a bug in the AMD IOMMU code. The early
detection code does not succeed if the GART is already detected. This patch
fixes this.

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