[media] s5p-mfc: use one implementation of s5p_mfc_get_new_ctx

Both version of MFC driver use functions with the same body and name.
The patch moves them to common location. It also simplifies it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>