msm: pm: Add mutex lock for shared data

The buffer allocated in file open operations need to be
protected as there can be a possiblity of use-after-free
scenario.

Process A              B
        |              |
      open             |
        |              |
      read started     |
        |             close

Add mutex lock to protect the buffer to avoid this.

Change-Id: I1e066fd0a047bcec2af7829015089d5c882fb2f3
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>
Signed-off-by: Naresh Malladi <namall@codeaurora.org>