drm: rcar-du: Lift z-pos restriction on primary plane for Gen3

There's no reason to require the primary plane to always be at the
bottom of the stack, as the VSP supports arbitrary ordering of planes,
and the KMS API doesn't have such a requirement either. Lift the
restriction.

As the primary plane can now be positioned arbitrarily, enable control
of its alpha channel as well.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Tomohito Esaki <etom@igel.co.jp>