iommu/vt-d: Use iova over first level

After we make all map/unmap paths support first level page table.
Let's turn it on if hardware supports scalable mode.
Signed-off-by: NLu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>

drivers/iommu/intel-iommu.c

@@ -1770,15 +1770,13 @@ static void free_dmar_iommu(struct intel_iommu *iommu)
 
 /*
  * Check and return whether first level is used by default for
- * DMA translation. Currently, we make it off by setting
- * first_level_support = 0, and will change it to -1 after all
- * map/unmap paths support first level page table.
+ * DMA translation.
  */
 static bool first_level_by_default(void)
 {
 	struct dmar_drhd_unit *drhd;
 	struct intel_iommu *iommu;
-	static int first_level_support = 0;
+	static int first_level_support = -1;
 
 	if (likely(first_level_support != -1))
 		return first_level_support;