[media] v4l: vsp1: uds: Enable scaling of alpha layer

Scaling of the alpha layer is disabled as both the RPF and WPF are
configured to hardcode the alpha value to 255. This results in a 0 alpha
value at the UDS output, making the image invisible when alpha blended
in the BRU. Fix it by enabling scaling of the alpha layer.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>