intel-iommu: fix build error with INTR_REMAP=y and DMAR=n

This fix should be safe since iommu->agaw is only used in intel-iommu.c.
And this file is only compiled with DMAR=y.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>