iommu: Move flush queue data into iommu_dma_cookie

Complete the move into iommu-dma by refactoring the flush queues
themselves to belong to the DMA cookie rather than the IOVA domain.

The refactoring may as well extend to some minor cosmetic aspects
too, to help us stay one step ahead of the style police.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/24304722005bc6f144e2a1fdd865d1465722fc2e.1639753638.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>