drm/tegra: dc: Dedicate overlay plane to cursor on older Tegra's

Older Tegra's do not support RGBA format for the cursor, but instead
overlay plane could be used for it. Since there is no much use for the
overlays on a regular desktop and HW-accelerated cursor is much better
than a SW cursor, let's dedicate one overlay plane to the mouse cursor.

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