drm/i915/gvt: Audit and shadow workload during ELSP writing

Let the workload audit and shadow ahead of vGPU scheduling, that
will eliminate GPU idle time and improve performance for multi-VM.

The performance of Heaven running simultaneously in 3VMs has
improved 20% after this patch.

v2:Remove condition current->vgpu==vgpu when shadow during ELSP
writing.

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