Revert "staging: binder: Change binder mutex to rtmutex."

This reverts commit 92ee040d ("staging: binder: Change binder
mutex to rtmutex."

The above change causes binder calls to take longer which negatively
affects performance.

Change-Id: I11eca50a45f7cce4a8b8eab9b5f428fe4e0b2b66
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>