media: v4l: vsp1: Setup BRU at atomic commit time

To implement fully dynamic plane assignment to pipelines, we need to
reassign the BRU and BRS to the DRM pipelines in the atomic commit
handler. In preparation for this setup factor out the BRU source pad
code and call it both at LIF setup and atomic commit time.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>