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: NLu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: NKevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20200724014925.15523-9-baolu.lu@linux.intel.comSigned-off-by: NJoerg Roedel <jroedel@suse.de>