iommu/tegra-smmu: Extract tegra_smmu_pte_get_use()

Extract the use count reference accounting into a separate function and
separate it from allocating the PTE.

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