msm: bam_dmux: remove obsolete mutex

The global mux lock is no longer shared between open/close and write
operations so there will not be any contention for the resource the lock
protected.  Thus remove the lock as it has no real purpose and introduces
performance reducing operations.

Change-Id: If9c6d64ecbbaae8563a3d69921849ffcfd3dc493
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>