drm/i915/gvt: Refine error handling for perform_bb_shadow

fix the wrong return type and return error once the unknown
command is scanned.

v2:
- separate this error handle from healthy rating code. (Zhenyu)

Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>