[media] v4l: vsp1: bru: Don't program background color 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 background color
in the control set handler is thus prohibited. Program it when starting
the module instead.

This requires storing the background color 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>