[media] omap3isp: preview: Rename last occurences of *_rgb_to_ycbcr to *_csc

For consistency reasons, rename the last remaining occurences of
rgb_to_ycbcr to csc (color space conversion).

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>