msm: vidc: optimize buffer alloc in bitrate check

Avoid alloc/free buffer at frame level in window based
bitrate check, re-use existing nodes to reduce
CPU workload, especially for HFR video.

Change-Id: I8bbbbef5229af7e608d7da1dfa4f75aab3903649
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>