drm/tegra: dc: Balance IOMMU group refcounting

Remove unneeded iommu_group_get() and add missing iommu_group_put(),
correcting IOMMU group refcount. This is a minor correction / cleanup that
doesn't really fix anything because Tegra's IOMMU driver are built-in and
hence groups refcounting can't hold IOMMU driver from unloading.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>