iommu/vt-d: Refactor device_to_iommu() helper

It is refactored in two ways:

- Make it global so that it could be used in other files.

- Make bus/devfn optional so that callers could ignore these two returned
values when they only want to get the coresponding iommu pointer.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20200724014925.15523-9-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

include/linux/intel-iommu.h

@@ -727,6 +727,7 @@ void iommu_flush_write_buffer(struct intel_iommu *iommu);
 int intel_iommu_enable_pasid(struct intel_iommu *iommu, struct device *dev);
 struct dmar_domain *find_domain(struct device *dev);
 struct device_domain_info *get_domain_info(struct device *dev);
+struct intel_iommu *device_to_iommu(struct device *dev, u8 *bus, u8 *devfn);
 
 #ifdef CONFIG_INTEL_IOMMU_SVM
 extern void intel_svm_check(struct intel_iommu *iommu);
@@ -765,8 +766,6 @@ struct intel_svm {
 	struct list_head devs;
 	struct list_head list;
 };
-
-extern struct intel_iommu *intel_svm_device_to_iommu(struct device *dev);
 #else
 static inline void intel_svm_check(struct intel_iommu *iommu) {}
 #endif