drm: sun4i: de2/de3: refactor mixer initialisation

drm: sun4i: de2/de3: refactor mixer initialisation

Now that the DE variant can be selected by enum, take the oppportunity
to factor out some common initialisation code to a separate function.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Ryan Walklin <ryan@testtoast.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20250528092431.28825-3-ryan@testtoast.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>