Flush queues are specific to DMA ops, which are now handled exclusively
by iommu-dma. As such, now that the historical artefacts from being
shared directly with drivers have been cleaned up, move the flush queue
code into iommu-dma itself to get it out of the way of other IOVA users.
This is pure code movement with no functional change; refactoring to
clean up the headers and definitions will follow.
Signed-off-by: Robin Murphy<robin.murphy@xxxxxxx>