drm/i915/selftests: Push the fake iommu device from the stack to data

Since we store a pointer to the fake iommu device that is allocated on
the stack, as soon as we leave the function it goes out of scope and any
future dereference is undefined behaviour. Just in case we may need to
look at the fake iommu device after initialiation, move the allocation
from the stack into the data.

Fixes: 01b9d4e2

 ("iommu/vt-d: Use dev_iommu_priv_get/set()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200916105022.28316-2-chris@chris-wilson.co.uk