drm/i915/gvt: Fix the incorrect length of child_device_config issue

GVT-g emualte the opregion for guest with bdb version as '186' which
child_device_config length should be '33'.

v2: split into 2 patch. 1st for issue fix, 2nd for code clean up.(Zhenyu)
v3: add fixes tag.(Zhenyu)

Fixes: 4023f301 ("drm/i915/gvt: opregion virtualization for win")
CC: Xiaolin Zhang <xiaolin.zhang@intel.com>
Reviewed-by: Xiaolin Zhang <xiaolin.zhang@intel.com>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>