msm: vidc: Increase minimum input buffer count for VP9 decode

Increase minimum input buffer count for VP9 decode to 6,as for
some vp9 clips which have superframes with more than 4 subframes
require more than 4 reference buffers to decode.

Change-Id: I561f4c3ad4c4a94c36293c26aab3a9c9423e9268
Signed-off-by: Deepak Kushwah <dkushwah@codeaurora.org>