It has been postulated that the pm_lock is bad for performance because
a CPU currently running rpmh_flush() could block other CPUs from
coming out of idle.  Similarly CPUs coming out of / going into idle
all need to contend with each other for the spinlock just to update
the variable tracking who's in PM.

Let's optimize this a bit.  Specifically:

- Use a count rather than a bitmask.  This is faster to access and
  also means we can use the atomic_inc_return() function to really
  detect who the last one to enter PM was.
- Accept that it's OK if we race and are doing the flush (because we
  think we're last) while another CPU is coming out of idle.  As long
  as we block that CPU if/when it tries to do an active-only transfer
  we're OK.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200504104917.v6.5.I295cb72bc5334a2af80313cbe97cb5c9dcb1442c@changeidSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>