drm/i915: to make vgpu ppgtt notificaiton as atomic operation

vgpu ppgtt notification was split into 2 steps, the first step is to
update PVINFO's pdp register and then write PVINFO's g2v_notify register
with action code to tirgger ppgtt notification to GVT side.

currently these steps were not atomic operations due to no any protection,
so it is easy to enter race condition state during the MTBF, stress and
IGT test to cause GPU hang.

the solution is to add a lock to make vgpu ppgtt notication as atomic
operation.

Cc: stable@vger.kernel.org
Signed-off-by: Xiaolin Zhang <xiaolin.zhang@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1566543451-13955-1-git-send-email-xiaolin.zhang@intel.com