drm/i915/selftests: Disable iommu for the mock device

On some machines, the iommu cannot allocate a domain for the mock device
causing the dma_map_sg() to fail, and the selftest to fail with -ENOMEM.
For the mock selftests, we are using a fake device and do not care about
iommu; so convince intel_iommu to treat us as a dummy device with an
identity mapping (and no iommu domain).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101080
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914162240.18310-1-chris@chris-wilson.co.uk
Tested-by: Elizabeth De La Torre Mena <elizabethx.de.la.torre.mena@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>