iommu/vt-d: Simplify include/linux/dmar.h

Simplify include/linux/dmar.h a bit based on the fact that
both CONFIG_INTEL_IOMMU and CONFIG_IRQ_REMAP select CONFIG_DMAR_TABLE.

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