media: verisilicon: Refactor postprocessor to store more buffers

Since vb2 queue can store more than VB2_MAX_FRAME buffers, the
postprocessor buffer storage must be capable to store more buffers too.
Change static dec_q array to allocated array to be capable to store
up to queue 'max_num_buffers'.
Keep allocating queue 'num_buffers' at queue setup time but also allows
to allocate postprocessors buffers on the fly.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
CC: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
CC: Philipp Zabel <p.zabel@pengutronix.de>