iommu/vt-d: Introduce helper domain_pfn_within_range() to simplify code

Introduce helper function domain_pfn_within_range() to simplify code
and improve readability.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>