drm/i915/gvt: remove unused type attributes

Only need to get attribute group instead of attributes and it has
no use, so remove it.

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191202070109.73924-1-zhenyuw@linux.intel.com