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

This reverts commit 5fc872c7.

The DMA-API does not strictly require that the memory
returned by dma_alloc_coherent is zeroed out. For that
another function (dma_zalloc_coherent) should be used. But
all other x86 DMA-API implementation I checked zero out the
memory, so that some drivers rely on it and break when it is
not.

It seems the (driver-)world is not yet ready for this
change, so revert it.

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