iommu/amd: Fix PPR interrupt processing logic

iommu/amd: Fix PPR interrupt processing logic

* Do not re-read ppr head pointer as its just updated by the driver.

* Do not read PPR buffer tail pointer inside while loop. If IOMMU
  generates PPR events continuously then completing interrupt processing
  takes long time. In worst case it may cause infinite loop.

Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20240418103400.6229-6-vasant.hegde@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>