drm/i915: Fix the kernel panic when using aliasing ppgtt

The ppgtt should be get directly from i915_address_space *vm instead of
vma->vm.

v2:
- add one more fix for bxt. (Chris)

Fixes: 4a234c5fae16 ("drm/i915: pass the vma to insert_entries")
Bugzilla:https://bugs.freedesktop.org/show_bug.cgi?id=101713
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com> v1
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1499421059-18262-1-git-send-email-chuanxiao.dong@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -910,7 +910,7 @@ static void gen8_ppgtt_insert_3lvl(struct i915_address_space *vm,
 				   enum i915_cache_level cache_level,
 				   u32 unused)
 {
-	struct i915_hw_ppgtt *ppgtt = i915_vm_to_ppgtt(vma->vm);
+	struct i915_hw_ppgtt *ppgtt = i915_vm_to_ppgtt(vm);
 	struct sgt_dma iter = {
 		.sg = vma->pages->sgl,
 		.dma = sg_dma_address(iter.sg),
@@ -2242,7 +2242,7 @@ static void bxt_vtd_ggtt_insert_entries__BKL(struct i915_address_space *vm,
 					     enum i915_cache_level level,
 					     u32 unused)
 {
-	struct insert_entries arg = { vma->vm, vma, level };
+	struct insert_entries arg = { vm, vma, level };
 
 	stop_machine(bxt_vtd_ggtt_insert_entries__cb, &arg, NULL);
 }