[media] v4l: omap4iss: Move code out of mutex-protected section

The pad::get_fmt call must be protected by a mutex, but preparing its
arguments doesn't need to be. Move the non-critical code out of the
mutex-protected section.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>