iommu/vt-d: Refactor first_level_by_default()

The first stage page table is compatible across host and guest kernels.
Therefore, this driver uses the first stage page table as the default for
paging domains.

The helper first_level_by_default() determines the feasibility of using
the first stage page table based on a global policy. This policy requires
consistency in scalable mode and first stage translation capability among
all iommu units. However, this is unnecessary as domain allocation,
attachment, and removal operations are performed on a per-device basis.

The domain type (IOMMU_DOMAIN_DMA vs. IOMMU_DOMAIN_UNMANAGED) should not
be a factor in determining the first stage page table usage. Both types
are for paging domains, and there's no fundamental difference between them.
The driver should not be aware of this distinction unless the core
specifies allocation flags that require special handling.

Convert first_level_by_default() from global to per-iommu and remove the
'type' input.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20241021085125.192333-7-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

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