drm/tegra: dc: Simplify atomic plane helper functions

Remove the tegra_dc_disable_window() function whose only purpose was to
allow tegra_plane_atomic_update() to also call it. Fix that by shuffling
tegra_plano_atomic_disable() to before tegra_plane_atomic_update().

While at it, also remove the overlay plane helper functions because they
are exactly the same as the primary plane helper functions.

Signed-off-by: Thierry Reding <treding@nvidia.com>