venus: helpers: extend set_num_bufs helper with one more argument

Extend venus_helper_set_num_bufs() helper function with one more
argument to set number of output buffers for the secondary decoder
output.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>