Atmel ISI support a preview path which can output RGB data.

So this patch introduces a bool variable to choose which path is
enabled currently. And also we need setup corresponding path registers.

By default the preview path is disabled. We only use Codec path.
Signed-off-by: NJosh Wu <josh.wu@atmel.com>
Signed-off-by: NGuennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>