drm/i915: Assert we do not attempt to reuse an allocated node

i915_gem_gtt_reserve() and i915_gem_gtt_insert() can only work on
unallocated nodes. Check that the callers complies.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170115172740.28995-1-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3627,6 +3627,7 @@ int i915_gem_gtt_reserve(struct i915_address_space *vm,
 	GEM_BUG_ON(!IS_ALIGNED(offset, I915_GTT_MIN_ALIGNMENT));
 	GEM_BUG_ON(range_overflows(offset, size, vm->total));
 	GEM_BUG_ON(vm == &vm->i915->mm.aliasing_ppgtt->base);
+	GEM_BUG_ON(drm_mm_node_allocated(node));
 
 	node->size = size;
 	node->start = offset;
@@ -3720,6 +3721,7 @@ int i915_gem_gtt_insert(struct i915_address_space *vm,
 	GEM_BUG_ON(start > 0  && !IS_ALIGNED(start, I915_GTT_PAGE_SIZE));
 	GEM_BUG_ON(end < U64_MAX && !IS_ALIGNED(end, I915_GTT_PAGE_SIZE));
 	GEM_BUG_ON(vm == &vm->i915->mm.aliasing_ppgtt->base);
+	GEM_BUG_ON(drm_mm_node_allocated(node));
 
 	if (unlikely(range_overflows(start, size, end)))
 		return -ENOSPC;