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: b5891fb520f7 ("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>

drivers/gpu/drm/i915/selftests/mock_gem_device.c

@@ -146,8 +146,10 @@ struct drm_i915_private *mock_gem_device(void)
 	dev_set_name(&pdev->dev, "mock");
 	dma_coerce_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
 
+#if IS_ENABLED(CONFIG_IOMMU_API)
 	/* hack to disable iommu for the fake device; force identity mapping */
 	pdev->dev.archdata.iommu = (void *)-1;
+#endif
 
 	dev_pm_domain_set(&pdev->dev, &pm_domain);
 	pm_runtime_enable(&pdev->dev);