drm/sun4i: mixer: Move layer enabling to atomic_update

drm/sun4i: mixer: Move layer enabling to atomic_update

Enable or disable layer only in layer atomic update callback. Doing so
will enable having separate layer driver later for DE33.

There is no fear that enable bit would be set incorrectly, as all
read-modify-write sequences for that register are now eliminated.

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/20251104180942.61538-11-jernej.skrabec@gmail.com
Signed-off-by: Chen-Yu Tsai <wens@kernel.org>