drm: sun4i: de2/de3: add mixer version enum

drm: sun4i: de2/de3: add mixer version enum

The Allwinner DE2 and DE3 display engine mixers are currently identified
by a simple boolean flag. This will not scale to support additional DE
variants.

Convert the boolean flag to an enum.

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/20250528092431.28825-2-ryan@testtoast.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>