[media] coda: implement forced key frames

Implement the V4L2_CID_MPEG_VIDEO_FORCE_KEY_FRAME control to force IDR
frames. This is useful to implement VFU (Video Fast Update) on RTP
transmissions.
We already force an IDR frame at the beginning of each GOP to work
around a firmware bug on i.MX27, use the same mechanism to service IDR
requests from userspace.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>