drm/i915/selftests: Only touch archdata.iommu when it exists

archdata.iommu only exists when CONFIG_IOMMU_API is enabled (and only
applies to intel-iommu in our case) so conditionally compile it out when
it doesn't exist.

Fixes: b5891fb5

 ("drm/i915/selftests: Disable iommu for the mock device")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170918164652.14200-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>