media: omap_vout: use struct v4l2_fh

This driver is one of the few that is still not using struct
v4l2_fh. Convert it.

Tested on a Pandaboard.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>