[media] omap3isp: preview: Add support for 8-bit formats at the sink pad

Support both grayscale (Y8) and Bayer (SBGGR8, SGBRG8, SGRBG8 and
SRGGB8) formats.

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>