drm/sun4i: layer: move num of planes calc out of layer code

drm/sun4i: layer: move num of planes calc out of layer code

With DE33, number of planes no longer depends on mixer because layers
are shared between all mixers.

Get this value via parameter, so DE specific code can fill in proper
value.

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/20251104180942.61538-16-jernej.skrabec@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>