drm/sun4i: Unify sun8i_*_layer structs

drm/sun4i: Unify sun8i_*_layer structs

These structs are identical, use a single struct to represent private
data for the DRM plane. This is a preparation for configuring layer
routing from the CRTC (mixer) instead of current approach of setting
up routing from individual layer's atomic_update callback.

Signed-off-by: Ondrej Jirman <megi@xff.cz>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20240224150604.3855534-2-megi@xff.cz
Signed-off-by: Maxime Ripard <mripard@kernel.org>