media: vimc: constify video_subdev structures

These structures are all only stored in fields of v4l2_subdev_ops
structures, all of which are const, so these structures can be const
as well.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>