iommu/arm-smmu-v3: Drop __GFP_ZERO flag from DMA allocation

Since commit 518a2f19 ("dma-mapping: zero memory returned from
dma_alloc_*"), dma_alloc_* always initializes memory to zero, so there
is no need to use dma_zalloc_* or pass the __GFP_ZERO flag anymore.

The flag was introduced by commit 04fa26c7 ("iommu/arm-smmu: Convert
DMA buffer allocations to the managed API"), since the managed API
didn't provide a dmam_zalloc_coherent() function.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>