drm/i915: Assert the vma's active tracking is clear before free

In looking at a use-after-free on Baytrail, it looks like the VMA's
activity tracking is suspect. Add some asserts to catch freeing the VMA
before we have decoupled all of its i915_gem_active trackers.

References: https://bugs.freedesktop.org/show_bug.cgi?id=101511
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620124321.1108-3-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>

drivers/gpu/drm/i915/i915_vma.c

@@ -579,11 +579,17 @@ int __i915_vma_do_pin(struct i915_vma *vma,
 
 static void i915_vma_destroy(struct i915_vma *vma)
 {
+	int i;
+
 	GEM_BUG_ON(vma->node.allocated);
 	GEM_BUG_ON(i915_vma_is_active(vma));
 	GEM_BUG_ON(!i915_vma_is_closed(vma));
 	GEM_BUG_ON(vma->fence);
 
+	for (i = 0; i < ARRAY_SIZE(vma->last_read); i++)
+		GEM_BUG_ON(i915_gem_active_isset(&vma->last_read[i]));
+	GEM_BUG_ON(i915_gem_active_isset(&vma->last_fence));
+
 	list_del(&vma->vm_link);
 	if (!i915_vma_is_ggtt(vma))
 		i915_ppgtt_put(i915_vm_to_ppgtt(vma->vm));
@@ -680,9 +686,8 @@ int i915_vma_unbind(struct i915_vma *vma)
 		__i915_vma_unpin(vma);
 		if (ret)
 			return ret;
-
-		GEM_BUG_ON(i915_vma_is_active(vma));
 	}
+	GEM_BUG_ON(i915_vma_is_active(vma));
 
 	if (i915_vma_is_pinned(vma))
 		return -EBUSY;