mm-video: update min buffer requirement for output buffers.

Currently in thumbnail mode min o/p buffer required is set to 1.
but decoder needs minimum two o/p buffers to display a frame
when low delay is 0 for any content for ex: XVID clips.
Update the minimum o/p buffers required to 2 for thumbnail mode
to handle such cases.

Change-Id: I09ec5945a3121d779247f836354731a162e0705d
1 file changed