video: msm: Remove buggy locking around FB operations

 * This code is unnecessary, and causes lockups at suspend if an
   operation is in progress and the mutex is taken. This happens
   frequently with the HDMI code, resulting in a DPM suspend timeout
   followed by a watchdog reset.

This reverts commit 8c189654223cca2d7cc6b9e1715385339917fae2.

Change-Id: I7e8bcdf5e6937240647a53e546e4f8445e86fd91