drm/sun4i: ui layer: Write attributes in one go

drm/sun4i: ui layer: Write attributes in one go

It turns out that none of the UI channel registers were meant to be
read. Mostly it works fine but sometimes it returns incorrect values.

Rework UI layer code to write all registers in one go to avoid reads.

This rework will also allow proper code separation.

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/20251104180942.61538-9-jernej.skrabec@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>