[media] coda: free context buffers under buffer mutex

Make sure the buffer_mutex lock is taken in coda_bit_release
while coda_free_framebuffers and coda_free_context_buffers
are called.

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>