drm/virtio: fix memory leak in virtio_gpu_cleanup_object()

Before setting shmem->pages to NULL, kfree() should
be called.

Signed-off-by: Xin He <hexin.op@bytedance.com>
Reviewed-by: Qi Liu <liuqi.16@bytedance.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200722051851.72662-1-hexin.op@bytedance.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>