iommu/vt-d, trivial: simplify code with existing macros

Simplify vt-d related code with existing macros and introduce a new
macro for_each_active_drhd_unit() to enumerate all active DRHD unit.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>