drm/sun4i: Add alpha property for sun8i UI layer

DE2.0 and DE3.0 UI layers supports plane-global alpha channel.
Add alpha property to the DRM plane and connect it to the
corresponding registers in mixer.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210128113940.347013-2-r.stratiienko@gmail.com