mm-video-v4l2: vdec: Retain lock while querying C2D filled length

On port reconfig, c2d is torn down & reopened. This can trigger a
race condition with any c2d call in the component context. Hold
the c2d lock while querying filled length to avoid this race
condition

Bug: 17692582

Author: Leena Winterrowd <lenhardw@codeaurora.org>
Change-Id: Icac85d41e5e39b510728537a980c7096d016358c