[media] s5p-mfc: correct the formats info for encoder

The NV12M is supported by all the version of MFC, so it is better
to use it as default OUTPUT format.
MFC v5 doesn't support NV21, I have tested it, for the SEC doc
it is not supported either.

Signed-off-by: ayaka <ayaka@soulik.info>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>