drm/i915/gvt: not to restore in-context mmio

Needn't to restore the in-context MMIO when SCHEDULE_OUT. Sometimes
with restoring the in-context MMIO, some GPU hang can be observed. So
remove the in-context MMIO restore

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>