drm/i915: Move assert of page pin vs bind count into i915_vma_unbind

The read of the page pin count and the bind count are unordered,
presenting races in the assert and it firing off incorrectly. Prevent
this by restricting the assert to the vma bind/unbind routines where we
have local cpu ordering between the two.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161231112012.29263-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -3291,7 +3291,6 @@ __i915_gem_object_unpin_pages(struct drm_i915_gem_object *obj)
 	GEM_BUG_ON(!obj->mm.pages);
 
 	atomic_dec(&obj->mm.pages_pin_count);
-	GEM_BUG_ON(atomic_read(&obj->mm.pages_pin_count) < obj->bind_count);
 }
 
 static inline void

drivers/gpu/drm/i915/i915_vma.c

@@ -640,6 +640,7 @@ int i915_vma_unbind(struct i915_vma *vma)
 	 * reaped by the shrinker.
 	 */
 	i915_gem_object_unpin_pages(obj);
+	GEM_BUG_ON(atomic_read(&obj->mm.pages_pin_count) < obj->bind_count);
 
 destroy:
 	if (unlikely(i915_vma_is_closed(vma)))