iommu/tegra: smmu: Remove unnecessary sanity check at alloc_pdir()

alloc_pdir() is called with smmu->as[?].pdir_page == NULL. No need to
check pdir_page again inside alloc_pdir().

Signed-off-by: Hiroshi DOYU <hdoyu@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>