iommu: Split off default domain allocation from group assignment

iommu: Split off default domain allocation from group assignment

When a bus is initialized with iommu-ops, all devices on the bus are
scanned and iommu-groups are allocated for them, and each groups will
also get a default domain allocated.

Until now this happened as soon as the group was created and the first
device added to it. When other devices with different default domain
requirements were added to the group later on, the default domain was
re-allocated, if possible.

This resulted in some back and forth and unnecessary allocations, so
change the flow to defer default domain allocation until all devices
have been added to their respective IOMMU groups.

The default domains are allocated for newly allocated groups after
each device on the bus is handled and was probed by the IOMMU driver.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200429133712.31431-12-joro@8bytes.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>