iommu/vt-d: Don't enable iommu's which have been ignored

The iommu driver will ignore some iommu units if there's no
device under its scope or those devices have been explicitly
set to bypass the DMA translation. Don't enable those iommu
units, otherwise the devices under its scope won't work.

Fixes: d8190dc6 ("iommu/vt-d: Enable DMA remapping after rmrr mapped")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>