drm/i915/gvt: Correct the privilege shadow batch buffer address

Once the ring buffer is copied to ring_scan_buffer and scanned,
the shadow batch buffer start address is only updated into
ring_scan_buffer, not the real ring address allocated through
intel_ring_begin in later copy_workload_to_ring_buffer.

This patch is only to set the right shadow batch buffer address
from Ring buffer, not include the shadow_wa_ctx.

v2:
- refine some comments. (Zhenyu)
v3:
- fix typo in title. (Zhenyu)
v4:
- remove the unnecessary comments. (Zhenyu)
- add comments in bb_start_cmd_va update. (Zhenyu)

Fixes: 0a53bc07f0 ("drm/i915/gvt: Separate cmd scan from request allocation")
Cc: stable@vger.kernel.org  # v4.15
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Yulei Zhang <yulei.zhang@intel.com>
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/scheduler.c

@@ -426,6 +426,17 @@ static int prepare_shadow_batch_buffer(struct intel_vgpu_workload *workload)
 			goto err;
 		}
 
+		/* For privilge batch buffer and not wa_ctx, the bb_start_cmd_va
+		 * is only updated into ring_scan_buffer, not real ring address
+		 * allocated in later copy_workload_to_ring_buffer. pls be noted
+		 * shadow_ring_buffer_va is now pointed to real ring buffer va
+		 * in copy_workload_to_ring_buffer.
+		 */
+
+		if (bb->bb_offset)
+			bb->bb_start_cmd_va = workload->shadow_ring_buffer_va
+				+ bb->bb_offset;
+
 		/* relocate shadow batch buffer */
 		bb->bb_start_cmd_va[1] = i915_ggtt_offset(bb->vma);
 		if (gmadr_bytes == 8)