drm/i915/gvt: move intel_runtime_pm_get out of spin_lock in stop_schedule

pm_runtime_get_sync in intel_runtime_pm_get might sleep if i915
device is not active. When stop vgpu schedule, the device may be
inactive. So need to move runtime_pm_get out of spin_lock/unlock.

Fixes: b24881e0("drm/i915/gvt: Add runtime_pm_get/put into gvt_switch_mmio
Cc: <stable@vger.kernel.org>
Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>