drm/i915/gvt: fix getting 64bit bar size error

For 64bit bar while reading the higher 32bit the value should be returned
directly.

In the current implementation the higher 32bit value was discarded and not
written to the cfg space of vgpu which lead to an incorrect bar size.

Signed-off-by: Xiaoguang Chen <xiaoguang.chen@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>