[media] omap_vout: Remove an obsolete comment

This comment mentions a field that doesn't exist, and talks about
videodev.h that got removed. So, it doesn't make any sense to keep
it.

Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>