drm/i915: Differentiate the aliasing_ppgtt with an invalid filp

Use an invalid filp so that the aliasing_ppgtt can be clearly
identified.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-22-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2380,7 +2380,7 @@ int i915_gem_init_aliasing_ppgtt(struct drm_i915_private *i915)
 	struct i915_hw_ppgtt *ppgtt;
 	int err;
 
-	ppgtt = i915_ppgtt_create(i915, NULL, "[alias]");
+	ppgtt = i915_ppgtt_create(i915, ERR_PTR(-EPERM), "[alias]");
 	if (IS_ERR(ppgtt))
 		return PTR_ERR(ppgtt);