iommu/tegra: smmu: modify pde/pte clean up

iommu/tegra: smmu: modify pde/pte clean up

when all ptes in the level 2 page tables need to be cleared, remove
the pde entry pointing to this page table first.

This way, even if page tables are being cleared while a HW is doing
a page walk, it will never endup in a decoding error at level 2 page
table. The fate of page walk is decided at level 1 page table itself.

Bug 200063337

Change-Id: Ia8121bf915783a9b7e551a92299664e5ed9a0f3b
Signed-off-by: Sri Krishna chowdary <schowdary@nvidia.com>
Reviewed-on: http://git-master/r/659675
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Hiroshi Doyu <hdoyu@nvidia.com>