msm: vdec: reduce decoder input buffer count for > 480fps case

For HFR(like 960 fps) usecases requires higher buffer count-24.
But due to framework change(277033), We don't need any addtional
buffers for HFR usecases also. So reducing the count back to 12.

Change-Id: Ia3669858cde0962275d00f31d2bbb7308a3ce983
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>