drm/i915/gvt: Add runtime_pm get/put to proctect MMIO accessing

In some cases, GVT-g is accessing MMIO without holding runtime_pm
and this patch can add the inline API for doing the runtime_pm get/put
to make sure when accessing HW MMIO the i915 HW is really powered on.

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