msm: msm_bus: Use RT mutexes for bus transactions

Change to using RT mutexes when processing a bus bandwidth transaction.
The bus bandwidth APIs are used by various processes of differing
priorities, use RT mutexes to prevent possible priority inversions in the
system. This change was made on the 3.10 branch as well.

Change-Id: Ic94d16b8782a11f556750a3e71e4e4d6bb5a9461
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>