drm: sun4i: de2/de3: add generic blender register reference function

drm: sun4i: de2/de3: add generic blender register reference function

The DE2 and DE3 engines have a blender register range within the
mixer engine register map, whereas the DE33 separates this out into
a separate display group.

Prepare for this by adding a function to look the blender reference up,
with a subsequent patch to add a conditional based on the DE type.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Ryan Walklin <ryan@testtoast.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20250528092431.28825-4-ryan@testtoast.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>