[bug report] drm/nouveau: use GEM references instead of TTMs

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Danilo Krummrich,

Commit bf32a3a12686 ("drm/nouveau: use GEM references instead of
TTMs") from Jul 18, 2024 (linux-next), leads to the following Smatch
static checker warning:

	drivers/gpu/drm/nouveau/dispnv50/disp.c:2818 nv50_display_destroy()
	error: we previously assumed 'disp->sync' could be null (see line 2816)

drivers/gpu/drm/nouveau/dispnv50/disp.c
    2804 static void
    2805 nv50_display_destroy(struct drm_device *dev)
    2806 {
    2807         struct nv50_disp *disp = nv50_disp(dev);
    2808 
    2809         nv50_audio_component_fini(nouveau_drm(dev));
    2810 
    2811         nvif_object_unmap(&disp->caps);
    2812         nvif_object_dtor(&disp->caps);
    2813         nv50_core_del(&disp->core);
    2814 
    2815         nouveau_bo_unmap(disp->sync);
    2816         if (disp->sync)
                     ^^^^^^^^^^
Check for NULL

    2817                 nouveau_bo_unpin(disp->sync);
--> 2818         nouveau_bo_fini(disp->sync);
                                 ^^^^^^^^^^
Unchecked dereference.  The patch didn't actually introduce this bug, it just
exposed it to the static checker.

    2819 
    2820         nouveau_display(dev)->priv = NULL;
    2821         kfree(disp);
    2822 }

regards,
dan carpenter