drm/rcar-du: Convert to the new generic alpha property

Now that we have support for per-plane alpha in the core, let's use it.

Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a343697b87109cd8d9675ea8bce2e561051a696f.1523432341.git-series.maxime.ripard@bootlin.com