drm/i915/gvt: Refine error handling for prepare_execlist_workload

refine the error handling for prepare_execlist_workload to restore to the
original states once error occurs.

only release the shadowed batch buffer and wa ctx when the workload is
completed successfully.

v2:
- split the mixed several error paths for better review. (Zhenyu)

v3:
- handle prepare batch buffer/wa ctx pin errors and
- emulate_schedule_in null issue. (Zhenyu)

v4:
- no need to handle emulate_schedule_in null issue. (Zhenyu)

v5:
- release the shadowed batch buffer and wa ctx only for the
  successful workload. (Zhenyu)

v6:
- polish the return style. (Zhenyu)

Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>