drm/i915/gvt: Do not reset pv_notified when vGPU transit from D3->D0

Unlike full initialization like normal boot, guest driver won't
pv_notified GVT when vGPU transit from D3->D0. If pv_notified is reset,
later vGPU operations will trigger enter into failsafe mode.

Considering the fact that vGPU will at least notify GVT pv_notified once
before D3/D0 transition, it's safe to skip reset pv_notified in D3->D0.

To test this feature, make sure S3 is enabled in QEMU parameters:
i440fx: PIIX4_PM.disable_s3=0
q35: ICH9-LPC.disable_s3=0
Also need enable sleep option in guest OS if it's disabled.

v2:
- Revise commit message to more accurate description. (Kevin)
- Split patch by logic. (Zhenyu)

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200709071002.247960-3-colin.xu@intel.com