drm/tegra: dc: Apply clipping to the plane

On Tegra20 an overlay plane should be clipped, otherwise its output is
distorted once plane crosses display boundary.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>