drm/sun4i: Start using layer id in DE2 driver

Till now, plane selection was hardcoded to first overlay in first UI
channel and layer parameter is unused.

Rename and add parameters to layer functions so they would represent HW
more accurately and start using then.

It turns out that overlays don't fit well in current DRM design, because
they can't be blended together or scaled independetly when they are set
to same channel. Because of that, always use only first overlay in each
channel.

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/20171201060550.10392-11-jernej.skrabec@siol.net