drm/i915/gvt: remove workload from intel_shadow_wa_ctx structure

intel_shadow_wa_ctx is a field of intel_vgpu_workload. container_of() can
be used to refine the relation-ship between intel_shadow_wa_ctx and
intel_vgpu_workload. This patch removes the useless dereference.

v2. add "drm/i915/gvt" prefix. (Zhenyu)

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>