iommu/tegra-smmu: Factor out tegra_smmu_set_pde()

This code is used both when creating a new page directory entry and when
tearing it down, with only the PDE value changing between both cases.

Factor the code out so that it can be reused.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[treding@nvidia.com: make commit message more accurate]
Signed-off-by: Thierry Reding <treding@nvidia.com>