drm/tegra: plane: Correct legacy blending

Keep old 'dependent' state of unaffected planes, this way new state takes
into account current state of unaffected planes.

Fixes: ebae8d07 ("drm/tegra: dc: Implement legacy blending")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>