[media] v4l: omap3isp: preview: Mark output buffer done first

The sequence number counter is incremented on each output buffer, and that
incremented value is used as the sequence number of that buffer. The input
buffer sequence numbering is based just on reading the same counter. If
the input buffer is marked done first, its sequence number ends up being
that of the output buffer - 1.

This is how the resizer works as well.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>