[media] coda: allocate per-context buffers from REQBUFS

Allocate the per-context buffers from REQBUFS instead in start_encoding or
start_decoding. This allows to stop and start streaming independently of
buffer (re)allocation

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>