drm/sun4i: backend: Add a custom plane state

We will need to store some additional data in the future to the state.
Create a custom plane state that will embed those data, in order to store
the pipe or whether or not that plane should use the frontend.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/88dd9c2b0caa550595e7b2ff37dc9d0af2c78609.1516613040.git-series.maxime.ripard@free-electrons.com