DE3.0 VI layers supports plane-global alpha channel.
DE2.0 FCC block have GLOBAL_ALPHA register that can be used as alpha source
for blender.

Add alpha property to the DRM plane and connect it to the
corresponding registers in the mixer.

Do not add alpha property for V3s SOC that have DE2.0 and 2 VI planes.
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/20210128113940.347013-3-r.stratiienko@gmail.com