iommu: Decouple iommu_present() from bus ops

Much as I'd like to remove iommu_present(), the final remaining users
are proving stubbornly difficult to clean up, so kick that can down the
road and just rework it to preserve the current behaviour without
depending on bus ops. Since commit 57365a04

 ("iommu: Move bus setup
to IOMMU device registration"), any registered IOMMU instance is already
considered "present" for every entry in iommu_buses, so it's simply a
case of validating the bus and checking we have at least once IOMMU.

Reviewed-by: Jason <Gunthorpe&lt;jgg@nvidia.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/caa93680bb9d35a8facbcd8ff46267ca67335229.1700589539.git.robin.murphy@arm.com


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