msm: vidc: Use registeredbuf.lock for buffer synchronization

Right now dynamic buffers are managed with both sync_lock and
registeredbufs.lock. sync_lock is meant only for state
transition. Hence remove sync_lock and achieve the same purpose
with registeredbufs.lock.

Change-Id: I3b0eb62019e7e992f63c159bf65d8c21cf2f52c8
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>