drm/i915/gvt: Add VM healthy check for submit_context

When a scan error occurs in submit_context, this patch is to
decrease the mm ref count and free the workload struct before
the workload is abandoned.

v2:
- submit_context related code should be combined together. (Zhenyu)

v3:
- free all the unsubmitted workloads. (Zhenyu)

v4:
- refine the clean path. (Zhenyu)

v5:
- polish the title. (Zhenyu)

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