libstagefright: OMAP4: Avoid buffer over-allocation

In some cases, mostly with flash videos, ducati buffer allocation
asks for a very high amount of buffers (seen 24 and 26). Limit the
max number of allocated buffers on output to 20 (the highest value
it actually accepts)

Change-Id: I473e5c8a39cf0c471a4478e0224e77f44ac71641