[media] v4l: vsp1: Implement atomic update for the DRM driver

Add two API functions named vsp1_du_atomic_begin() and
vsp1_du_atomic_flush() to signal the start and end of an atomic update.
The vsp1_du_setup_rpf() function is renamed to vsp1_du_atomic_update()
for consistency.

With this new API, the driver will reprogram all modified inputs
atomically before restarting the video stream.

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