msm: vidc: Acquire lock before iterating over vb2_bufq list

In msm_comm_flush_in_invalid_state, the vb2_bufq list
was iterated over without holding the bufq lock. This
can lead to a race condition and corrupt the list, if we get an
EBD or FBD while flushing the buffers. This change helps avoid
any race condition while iterating over the vb2_bufq list.

Change-Id: I7be939aec0c7bd274a211c29537607f695758bfc
Signed-off-by: Arun Menon <avmenon@codeaurora.org>