drm/i915/gvt: Free dmabuf_obj list in intel_vgpu_dmabuf_cleanup

The per vGPU dmabuf_obj list should be released in intel_vgpu_dmabuf_
cleanup, which is invoked either in the process of closing a VM or in
the process of removing a vGPU.

Fixes: e3a0d7976c53 ("drm/i915/gvt: Handle orphan dmabuf_objs")
Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>