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 <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67287
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/i915/i915_dma.c

···
 	spin_lock_init(&dev_priv->gpu_error.lock);
 	spin_lock_init(&dev_priv->backlight.lock);
 	spin_lock_init(&dev_priv->uncore.lock);
+	spin_lock_init(&dev_priv->mm.object_stat_lock);
 	mutex_init(&dev_priv->dpio_lock);
 	mutex_init(&dev_priv->rps.hw_lock);
 	mutex_init(&dev_priv->modeset_restore_lock);

drivers/gpu/drm/i915/i915_drv.h

···
 	struct drm_i915_gem_phys_object *phys_objs[I915_MAX_PHYS_OBJECT];
 
 	/* accounting, useful for userland debugging */
+	spinlock_t object_stat_lock;
 	size_t object_memory;
 	u32 object_count;
 };

drivers/gpu/drm/i915/i915_gem.c

···
 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