mm / iommu: include need_iommu() test in iommu_use_hap_pt()

mm / iommu: include need_iommu() test in iommu_use_hap_pt()

The name 'iommu_use_hap_pt' suggests that that P2M table is in use as the
domain's IOMMU pagetable which, prior to this patch, is not strictly true
since the macro did not test whether the domain actually has IOMMU
mappings.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>