drm/sun4i: de2/de3: Move plane type determination to mixer

drm/sun4i: de2/de3: Move plane type determination to mixer

Plane type determination logic inside layer init functions doesn't allow
index register to be repurposed to plane sequence, which it almost is.

So move out the logic to mixer, which allows further rework for DE33
support.

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/20251104180942.61538-14-jernej.skrabec@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>