drm/i915: fix the racy object accounting

Just use a spinlock to protect them.

v2: Rebase onto the new object create refcount fix patch.

v3: Don't kill dev_priv->mm.object_memory as requested by Chris and
hence just use a spinlock instead of atomic_t.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67287
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index eceab96..35d17fb 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -75,15 +75,19 @@
 static void i915_gem_info_add_obj(struct drm_i915_private *dev_priv,
 				  size_t size)
 {
+	spin_lock(&dev_priv->mm.object_stat_lock);
 	dev_priv->mm.object_count++;
 	dev_priv->mm.object_memory += size;
+	spin_unlock(&dev_priv->mm.object_stat_lock);
 }
 
 static void i915_gem_info_remove_obj(struct drm_i915_private *dev_priv,
 				     size_t size)
 {
+	spin_lock(&dev_priv->mm.object_stat_lock);
 	dev_priv->mm.object_count--;
 	dev_priv->mm.object_memory -= size;
+	spin_unlock(&dev_priv->mm.object_stat_lock);
 }
 
 static int