Allocating the bitstream buffer only when the format is set allows to guarantee
that at least two frames fit into the bitstream buffer. For small frame sizes
a smaller bitstream buffer can be allocated. Since the bitstream buffer size now
depends on the format, replace CODA_MAX_FRAME_SIZE with ctx->bitstream.size
where appropriate and remove the now unused constant.
Since REQBUFS can be called multiple times, but the format can't be changed
unless REQBUFS 0 was called before, we can just keep the allocated context and
bitstream buffers if REQBUFS is called multiple times with a non-zero buffer
count.

[fixed a resource leak preventing repeatedly decoding]
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
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>