drm/sun4i: Move blender config from layers to mixer

drm/sun4i: Move blender config from layers to mixer

With upcoming DE33 support, layer management must be decoupled from
other operations like blender configuration. There are two reasons:
- DE33 will have separate driver for planes and thus it will be harder
  to manage different register spaces
- Architecturaly it's better to split access by modules. Blender is now
  exclusively managed by mixer.

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/20251104180942.61538-8-jernej.skrabec@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>