iommu/tegra-smmu: Use __GFP_ZERO to allocate zeroed pages

Rather than explicitly zeroing pages allocated via alloc_page(), add
__GFP_ZERO to the gfp mask to ask the allocator for zeroed pages.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Thierry Reding <treding@nvidia.com>