[media] s5p-mfc: Add handling of buffer freeing reqbufs request

The encoder forget the work to call hardware to release its buffers.
This patch came from chromium project. I just change its code
style and make the API match with new kernel.

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>