[media] omap3isp: preview: Fix output size computation depending on input format

The preview engine crops 4 columns and 4 lines when CFA is enabled.
Commit b2da46e5 ("omap3isp: preview: Add
support for greyscale input") inverted the condition by mistake, fix
this.

Reported-by: Florian Neuhaus <florian.neuhaus@reberinformatik.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Florian Neuhaus <florian.neuhaus@reberinformatik.ch>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>