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

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: 0a53bc07f044 ("drm/i915/gvt: Separate cmd scan from request allocation")
Cc: [email protected]  # v4.15
Cc: Zhenyu Wang <[email protected]>
Cc: Yulei Zhang <[email protected]>
Signed-off-by: fred gao <[email protected]>
Signed-off-by: Zhenyu Wang <[email protected]>