[IA64] enable setting DMAR on by default

The previous commit which introduced the DMAR_DEFAULT_ON setting in
drivers/pci/dmar.c neglected to add the ability for ia64 to enable
the IOMMU by default. Rectify that mistake, doh!

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>