drm/i915: Assert that we never create a vma for the aliasing_ppgtt

The aliasing_ppgtt is just a container for the HW context that mirrors
the global gtt. It should never be used directly, so assert if we make
the mistake of trying to allocate a VMA for it.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170209111933.12420-1-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_vma.c

@@ -78,6 +78,9 @@ vma_create(struct drm_i915_gem_object *obj,
 	struct rb_node *rb, **p;
 	int i;
 
+	/* The aliasing_ppgtt should never be used directly! */
+	GEM_BUG_ON(vm == &vm->i915->mm.aliasing_ppgtt->base);
+
 	vma = kmem_cache_zalloc(vm->i915->vmas, GFP_KERNEL);
 	if (vma == NULL)
 		return ERR_PTR(-ENOMEM);