iommu: Move dev_iommu_ops() to private header

iommu: Move dev_iommu_ops() to private header

dev_iommu_ops() is essentially only used in iommu subsystem, so move to a
private header to avoid being abused by other drivers.

Link: https://lore.kernel.org/r/20230818101033.4100-2-yi.l.liu@intel.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>