iommu/vt-d: Refactor device_to_iommu() to retrieve iommu directly

iommu/vt-d: Refactor device_to_iommu() to retrieve iommu directly

The device_to_iommu() helper was originally designed to look up the DMAR
ACPI table to retrieve the iommu device and the request ID for a given
device. However, it was also being used in other places where there was
no need to lookup the ACPI table at all.

Retrieve the iommu device directly from the per-device iommu private data
in functions called after device is probed.

Rename the original device_to_iommu() function to a more meaningful name,
device_lookup_iommu(), to avoid mis-using it.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20231116015048.29675-2-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>