drm/i915/gvt: Fix orphan vgpu dmabuf_objs' lifetime

Deleting dmabuf item's list head after releasing its container can lead
to KASAN-reported issue:

  BUG: KASAN: use-after-free in __list_del_entry_valid+0x15/0xf0
  Read of size 8 at addr ffff88818a4598a8 by task kworker/u8:3/13119

So fix this issue by puting deleting dmabuf_objs ahead of releasing its
container.

Fixes: dfb6ae4e

 ("drm/i915/gvt: Handle orphan dmabuf_objs")
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200225053527.8336-2-tina.zhang@intel.com