[media] v4l2-ioctl: don't set PRIV_MAGIC unconditionally in g_fmt()

Regression fix:

V4L2_PIX_FMT_PRIV_MAGIC should only be set for the VIDEO_CAPTURE and
VIDEO_OUTPUT buffer types, and not for any others. In the case of
the win format this overwrote a pointer value that is passed in from
userspace.

Just set it for V4L2_BUF_TYPE_VIDEO_CAPTURE and OUTPUT only. Set
it before the callback is called, just as is done for try/s_fmt, and
again afterwards in case the driver zeroed it. The latter was missing
in try/s_fmt, so add it there as well. Currently it is quite likely
that drivers clear priv (that was needed for a long time), so it makes
sense to set it twice.

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