msm: vidc: fix dec out buf count update logic for batching mode

When update video decoder output buffer count, only considering
inst->decode_batching is not enough, should use is_batching_allowed
function to judge.

This patch will decrease output buffer count for some streams,
because is_batching_allowed() function has more restriction to
enable batching mode, while batching mode need additional output
buffer.

Change-Id: I24c9883c9f5a9e3ec9857ba9e0b4468e425b7e77
Signed-off-by: Bo Fang <bofang1@codeaurora.org>