iommu/amd: Rearrange device flush code

Consolidate all flush related code in one place so that its easy
to maintain.

No functional changes intended.

Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20240205115615.6053-16-vasant.hegde@amd.com


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