drm/i915: Don't unconditionally try to deref aliasing ppgtt

Since the beginning, the functions which try to properly reference the
aliasing PPGTT have deferences a potentially null aliasing_ppgtt member.
Since the accessors are meant to be global, this will not do.

Introduced originally in:
commit a70a3148b0c61cb7c588ea650db785b261b378a3
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Wed Jul 31 16:59:56 2013 -0700

    drm/i915: Make proper functions for VMs

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -4975,7 +4975,8 @@ unsigned long i915_gem_obj_offset(struct drm_i915_gem_object *o,
 	struct drm_i915_private *dev_priv = o->base.dev->dev_private;
 	struct i915_vma *vma;
 
-	if (vm == &dev_priv->mm.aliasing_ppgtt->base)
+	if (!dev_priv->mm.aliasing_ppgtt ||
+	    vm == &dev_priv->mm.aliasing_ppgtt->base)
 		vm = &dev_priv->gtt.base;
 
 	BUG_ON(list_empty(&o->vma_list));
@@ -5016,7 +5017,8 @@ unsigned long i915_gem_obj_size(struct drm_i915_gem_object *o,
 	struct drm_i915_private *dev_priv = o->base.dev->dev_private;
 	struct i915_vma *vma;
 
-	if (vm == &dev_priv->mm.aliasing_ppgtt->base)
+	if (!dev_priv->mm.aliasing_ppgtt ||
+	    vm == &dev_priv->mm.aliasing_ppgtt->base)
 		vm = &dev_priv->gtt.base;
 
 	BUG_ON(list_empty(&o->vma_list));