[media] omap3isp: preview: Lower the crop margins

The preview engine includes filters that consume columns and lines as
part of their operation, thus resulting in a cropped image. To allow
turning those filters on/off during streaming without affecting the
output image size, the driver adds additional cropping to make the total
number of cropped columns and lines constant regardless of which filters
are enabled.
This process needlessly includes the CFA filter, as whether the filter
is enabled only depends on the sink pad format, which can't change
during streaming.
Exclude the CFA filter from the preview engine margins.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>