iommu/tegra-smmu: Fix page table lookup in unmap/iova_to_phys methods

Fix the page table lookup in the unmap and iova_to_phys methods.
Neither of these methods should allocate a page table; a missing page
table should be treated the same as no mapping present.

More importantly, using as_get_pte() for an IOVA corresponding with a
non-present page table entry increments the use-count for the page
table, on the assumption that the caller of as_get_pte() is going to
setup a mapping.  This is an incorrect assumption.

Fix both of these bugs by providing a separate helper which only looks
up the page table, but never allocates it.  This is akin to pte_offset()
for CPU page tables.

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