[media] s5p-mfc: add the support of V4L2_CID_MPEG_VIDEO_FORCE_KEY_FRAME

There is a new control V4L2_CID_MPEG_VIDEO_FORCE_KEY_FRAME to
force an encoder key frame. It is the same as requesting
V4L2_MPEG_MFC51_VIDEO_FORCE_FRAME_TYPE_I_FRAME.

Signed-off-by: Wu-Cheng Li <wuchengli@chromium.org>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>