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: 4a234c5f ("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>