msm: vidc: Update buffer couting logic for PMS

Today, driver counts the buffers with the client based on vb state
and bytesused. This logic also accouting the buffers which were
never queued to driver. By doing this, driver wrongly adjusting PMS.
Fix the mismatch by using DECODEONLY_FLAG.

CRs-Fixed: 2142478
Change-Id: I9802102de44f7668d1c4d11f8636348695acea47
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>