[media] omap3isp: preview: Fix contrast and brightness handling

Commit bac387efbb88cf0e8df6f46a38387897cea464ee ("omap3isp: preview:
Simplify configuration parameters access") added three fields to the
preview_update structure, but failed to properly update the related
initializers. Fix this.

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