iommu/vt-d: Move common code out of iommu_attch_device()

This part of code could be used by both normal and aux
domain specific attach entries. Hence move them into a
common function to avoid duplication.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>