media: coda: assert bitstream mutex is locked in coda_fill_bitstream

coda_fill_bitstream() must be called under the bitstream mutex.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>