As we use a mutex to serialise the first acquire (as it may be a lengthy
operation), but only an atomic decrement for the release, we have to
be careful in case a second thread races and completes both
acquire/release as the first finishes its acquire.

Thread A			Thread B
i915_active_acquire		i915_active_acquire
  atomic_read() == 0		  atomic_read() == 0
  mutex_lock()			  mutex_lock()
				  atomic_read() == 0
				    ref->active();
				  atomic_inc()
				  mutex_unlock()
  atomic_read() == 1
				i915_active_release
				  atomic_dec_and_test() -> 0
				    ref->retire()
  atomic_inc() -> 1
  mutex_unlock()

So thread A has acquired the ref->active_count but since the ref was
still active at the time, it did not initialise it. By switching the
check inside the mutex to an atomic increment only if already active, we
close the race.

Fixes: c9ad602f ("drm/i915: Split i915_active.mutex into an irq-safe spinlock for the rbtree")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200126102346.1877661-3-chris@chris-wilson.co.uk