drm/sun4i: Rework enabling plane in DE2 driver

If we want to support multiple planes in the future, code which enables
pipe has to be moved to appropriate place and it must depend on channel
id instead of being hardcoded.

Side effect of that rework is definition of default Z position. For now,
put first channel at the bottom, second above it and so on.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-10-jernej.skrabec@siol.net