media: coda: always hold back decoder jobs until we have enough bitstream payload

The bitstream prefetch unit reads data in 256 byte blocks with some kind
of queueing. For the decoder to see data up to a desired position in the
next run, the bitstream has to be filled for 2 256 byte blocks past that
position aligned up to the next 256 byte boundary.
This should make sure we never run into a buffer underrun condition if
userspace does not supply new input buffers fast enough.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>