utils: libtuning: Correct GBRG Image parsing

The Image class incorrectly parses data in GBRG bayer formats as the
indices to the channels are set incorrectly - fix it.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>