iommu/tegra: smmu: fix issue with detach_dev

iommu/tegra: smmu: fix issue with detach_dev

While detaching a device, make sure the debugfs_root of client
is marked as NULL. In case the device re-attaches itself with SMMU,
the stale pointer from debugfs_root will be used to create debugfs
and can cause kernel panic.

Bug 1546169
Bug 200031739

Change-Id: If0f58e4106ca2fe4b761c7fe185b17272824f76d
Signed-off-by: Sri Krishna chowdary <schowdary@nvidia.com>
Reviewed-on: http://git-master/r/486335
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Hiroshi Doyu <hdoyu@nvidia.com>
Tested-by: Hiroshi Doyu <hdoyu@nvidia.com>