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

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

Signed-off-by: Deepak Kushwah <dkushwah@codeaurora.org>
Signed-off-by: Surajit Podder <spodder@codeaurora.org>
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>
Change-Id: I561f4c3ad4c4a94c36293c26aab3a9c9423e9268