[media] coda: rework meta counting and add separate lock

Keep count of number of buffer meta structures in the list and use
a separate spinlock for operations on this counted list instead
of reusing the bitstream mutex in some places and none at all in
others.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>