drm/i915/gvt: make width of mmio_attribute bigger

8 bits are all used up. extend it to 16 bits to hold more flags.

Suggested-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201223034545.17224-1-yan.y.zhao@intel.com


Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>