Revert "i915: Defer final wakeup on active until after retirement."

This reverts i915_active.c 1.11.  ref->retire might free the object,
so touching it is not allowed -- that would use-after-free.  Linux
uses the object only for its address with wake_up_var.

The reason I made this change was that I guessed i915_active_wait has
to wait until after ref->retire finishes -- after all, Linux seems to
defer the wakeup until then.  However, even the Linux code doesn't
guarantee this, because i915_active_wait could be called _during_
ref->retire, and would witness ref->count == 0, and would not wait
until it has completed in that case.  So maybe my guess was wrong,
and it is OK for i915_active_wait to return while ref->retire is
still in flight -- I don't see any logic that obviously requires it
to wait for ref->retire, in any case.

Or maybe something does rely on i915_active_wait to wait for
ref->retire to finish, in which case we need a different mechanism
for i915_active_release itself to wait until i915_active_retire has
woken up, without dereferencing ref since it might be dead after
ref->retire.

(riastradh)