[media] v4l: vsp1: Set the alpha value manually in RPF and WPF s_stream handlers

The RPF and WPF alpha values are set through V4L2 controls and applied
when starting the video stream by a call to v4l2_ctrl_handler_setup().
As that function uses the control handler mutex it can't be called in
interrupt context, where the VSP+DU pipeline handler might need to
reconfigure the pipeline.

Set the alpha value manually in the RPF and WPF s_stream handler to
ensure that the hardware is properly configured even when controlled
without the userspace API. If the userspace API is enabled protect that
with the control lock to avoid race conditions with userspace.

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