Comment 12 for bug 1688158

This is fixed in upstream by patch, will cherry-pick this patch and test on intel system to fix this regression, and resubmit the sru.

commit 0ef7b235aeddf9b2f78889c7823a24745ebf0cf7
Author: Andy Shevchenko <email address hidden>
Date: Wed Feb 15 16:42:21 2017 +0200

    iommu/vt-d: Fix crash on boot when DMAR is disabled

    By default CONFIG_INTEL_IOMMU_DEFAULT_ON is not set and thus
    dmar_disabled variable is set.

    Intel IOMMU driver based on above doesn't set intel_iommu_enabled
    variable.

    The commit b0119e870837 ("iommu: Introduce new 'struct iommu_device'")
    mistakenly assumes it never happens and tries to unregister not ever
    registered resources, which crashes the kernel at boot time: