iommu/amd: Don't allocate with __GFP_ZERO in alloc_coherent

Don't explicitly add __GFP_ZERO to the allocator flags.
Leave this up to the caller.

Tested-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>