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 b0119e87 ("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:

	BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
	IP: iommu_device_unregister+0x31/0x60

Make unregister procedure conditional in free_iommu().

Fixes: b0119e87 ("iommu: Introduce new 'struct iommu_device'")
Cc: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/dmar.c

@@ -1108,8 +1108,10 @@ static int alloc_iommu(struct dmar_drhd_unit *drhd)
 
 static void free_iommu(struct intel_iommu *iommu)
 {
-	iommu_device_sysfs_remove(&iommu->iommu);
-	iommu_device_unregister(&iommu->iommu);
+	if (intel_iommu_enabled) {
+		iommu_device_unregister(&iommu->iommu);
+		iommu_device_sysfs_remove(&iommu->iommu);
+	}
 
 	if (iommu->irq) {
 		if (iommu->pr_irq) {