To perform the workload scan and shadow in ELSP writing stage for
performance consideration, the workload scan and shadow stuffs
should be factored out from dispatch_workload().

v2:Put context pin before i915_add_request;
   Refine the comments;
   Rename some APIs;

v3:workload->status should set only when error happens.
v4:i915_add_request is must to have after i915_gem_request_alloc.
Signed-off-by: NPing Gao <ping.a.gao@intel.com>
Reviewed-by: NZhi Wang <zhi.a.wang@intel.com>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>