drm/i915: Avoid refcount_inc on known zero count

In intel_wakeref_auto, we use refcount_inc_not_zero to detect the first
use and initialise the timer. On doing so, we have to avoid using
refcount_inc on that zero count as the debug code flags that as an
error:
	refcount_t: increment on 0; use-after-free.

Rearrange the code so that if we know the count is 0 and we are
initialising, we explicitly set it to 1.

Fixes: b27e35ae ("drm/i915: Keep user GGTT alive for a minimum of 250ms")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528154053.22004-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_wakeref.c

@@ -114,11 +114,11 @@ void intel_wakeref_auto(struct intel_wakeref_auto *wf, unsigned long timeout)
 
 	if (!refcount_inc_not_zero(&wf->count)) {
 		spin_lock_irqsave(&wf->lock, flags);
-		if (!refcount_read(&wf->count)) {
+		if (!refcount_inc_not_zero(&wf->count)) {
 			GEM_BUG_ON(wf->wakeref);
 			wf->wakeref = intel_runtime_pm_get_if_in_use(wf->i915);
+			refcount_set(&wf->count, 1);
 		}
-		refcount_inc(&wf->count);
 		spin_unlock_irqrestore(&wf->lock, flags);
 	}