iommu/amd: Tidy up DMA ops init

Now that DMA ops are part of the core API via iommu-dma, fold the
vestigial remains of the IOMMU_DMA_OPS init state into the IOMMU API
phase, and clean up a few other leftovers. This should also close the
race window wherein bus_set_iommu() effectively makes the DMA ops state
visible before its nominal initialisation - it seems this was previously
fairly benign, but since commit a250c23f ("iommu: remove
DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE") it can now lead to the strict flush
queue policy inadvertently being picked for default domains allocated
during that window, with a corresponding unexpected perfomance impact.

Reported-by: Jussi Maki <joamaki@gmail.com>
Tested-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Fixes: a250c23f ("iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE")
Link: https://lore.kernel.org/r/665db61e23ff8d54ac5eb391bef520b3a803fcb9.1622727974.git.robin.murphy@arm.com


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