msm: vidc: Tune input buffer count for performance

With just two input buffers, video hardware stays
idle quite often waiting for data. With the new
input buffer count venus idle time is reduced and
improved performance is observed.

Change-Id: Ia44e106e4c8cf46ebcae9c89054508337c23a6cb
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>