media: venus: vdec_ctrls: get real minimum buffers for capture

Until now we returned num_output_bufs set during reqbuf but
that could be wrong when we implement stateful Codec API. So
get the minimum buffers for capture from HFI. This is supposed
to be called after stream header parsing, i.e. after dequeue
v4l2 event for change resolution.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>