drm/sun4i: ui_layer: Change index meaning

drm/sun4i: ui_layer: Change index meaning

In the pursuit of making UI/VI layer code independent of DE version,
change meaning of UI index to index of the plane within mixer. DE33 can
split amount of VI and UI planes between multiple mixer in whatever way
it deems acceptable, so simple calculation VI num + UI index won't be
meaningful anymore.

Reviewed-by: Chen-Yu Tsai <wens@kernel.org>
Tested-by: Ryan Walklin <ryan@testtoast.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://patch.msgid.link/20251104180942.61538-15-jernej.skrabec@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>