[media] v4l: vsp1: rwpf: Don't program alpha value in control set handler

The datasheet clearly states that all but a few registers can't be
modified when the device is running. Programming the alpha value in
the control set handler is thus prohibited. Program it when starting the
module instead.

This requires storing the alpha value internally as the module can be
started from the frame completion interrupt handler, and accessing
control values requires taking a mutex.

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