drm/i915/gvt: clean up unsubmited workloads before destroying kmem cache

drm/i915/gvt: clean up unsubmited workloads before destroying kmem cache

This is to fix a memory leak issue caused by unfreed gvtg workload objects.
Walk through the workload list and free all of the remained workloads
before destroying kmem cache.

[179.885211] INFO: Object 0xffff9cef10003b80 @offset=7040
[179.885657] kmem_cache_destroy gvt-g_vgpu_workload: Slab cache still has objects
[179.886146] CPU: 2 PID: 2318 Comm: win_lucas Tainted: G    B   W       4.11.0+ #1
[179.887223] Call Trace:
[179.887394]  dump_stack+0x63/0x90
[179.887617]  kmem_cache_destroy+0x1cf/0x1e0
[179.887960]  intel_vgpu_clean_execlist+0x15/0x20 [i915]
[179.888365]  intel_gvt_destroy_vgpu+0x4c/0xd0 [i915]
[179.888688]  intel_vgpu_remove+0x2a/0x30 [kvmgt]
[179.888988]  mdev_device_remove_ops+0x23/0x50 [mdev]
[179.889309]  mdev_device_remove+0xe4/0x190 [mdev]
[179.889615]  remove_store+0x7d/0xb0 [mdev]
[179.889885]  dev_attr_store+0x18/0x30
[179.890129]  sysfs_kf_write+0x37/0x40
[179.890371]  kernfs_fop_write+0x107/0x180
[179.890632]  __vfs_write+0x37/0x160
[179.890865]  ? kmem_cache_alloc+0xd7/0x1b0
[179.891116]  ? apparmor_file_permission+0x1a/0x20
[179.891372]  ? security_file_permission+0x3b/0xc0
[179.891628]  vfs_write+0xb8/0x1b0
[179.891812]  SyS_write+0x55/0xc0
[179.891992]  entry_SYSCALL_64_fastpath+0x1e/0xad

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>