drm/i915/gvt: To check whether workload scan and shadow has mutex hold

The function workload scan and shadow have to hold the drm.struct_mutex
before called. To avoid misusing of this function, add a lockdep assert
in it.

Signed-off-by: Ping Gao <ping.a.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>