[media] coda: allocate bitstream ringbuffer only for BIT decoder

The BIT encoder does not use a per-context bitstream ringbuffer as it encodes
directly into the videobuf2 capture queue's buffers. Avoid allocation of the
bitstream ringbuffer for encoder contexts.

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>