[media] coda: store bitstream buffer position with buffer metadata

Storing the buffer position in the bitstream with the buffer metadata
allows to later use that information to drop metadata for skipped buffers
and to determine whether bitstream padding has to be applied.

This patch also renames struct coda_timestamp to struct coda_buffer_meta
to make clear that it contains more than only the buffer timestamp.

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>