iommu: Remove iommu_present()

The last callsite of iommu_present() is removed by commit <45c690ae>
("drm/tegra: Use iommu_paging_domain_alloc()"). Remove it to avoid dead
code.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20241009051808.29455-1-baolu.lu@linux.intel.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>