drm/i915/gvt: fix lock not released bug for dispatch_workload() err path

Need to be careful to release struct_mutext when request alloc
failed and take consistent handling for return status as with
normal go out path. Ensure to check correct workload request in
complete path too.

v2: Add Fixes note

Fixes: 90d27a1b ("drm/i915/gvt: fix deadlock in workload_thread")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>