drm/tegra: plane: Support 180° rotation

Combining horizontal and vertical reflections gives us 180 degrees of
rotation. Both reflection modes are already supported, and thus, we just
need to mark the 180 rotation mode as supported. The 180 rotation mode is
needed for devices like Nexus 7 tablet, which have display panel mounted
upside-down.

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