mako: powerd hangs after rapid screen on/off when using compat sf

Pressing the power button rapidly on mako to make the screen turn on and off quickly will reliably trigger a hang on the powerd main thread. Debugging revealed that the last function called by powerd prior to the hang is either sf_blank() or sf_unblank(), which are never returning. gdb shows that the top of the call stack is somewhere within Android's libc, with /proc/pid/wchan showing that the thread is blocked in the kernel within binder_thread_read().