msm: vidc: Halve input buffer size during secure playback

Since we have limited carveout memory, in order to have sufficient
memory for dual secure playback, input buffer size is halved.

Change-Id: I733934ebbb85fbfa685ab4080a05d27e5478709c
Signed-off-by: Akshay Chandrashekhar Kalghatgi <akalghat@codeaurora.org>