drm/nv50-nvc0: make sure vma is definitely unmapped when destroying bo

Somehow fixes a misrendering + hang at GDM startup on my NVA8...

My first guess would have been stale TLB entries laying around that a new
bo then accidentally inherits.  That doesn't make a great deal of sense
however, as when we mapped the pages for the new bo the TLBs would've
gotten flushed anyway.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -49,7 +49,10 @@ nouveau_bo_del_ttm(struct ttm_buffer_object *bo)
 		DRM_ERROR("bo %p still attached to GEM object\n", bo);
 
 	nv10_mem_put_tile_region(dev, nvbo->tile, NULL);
+	if (nvbo->vma.node) {
+		nouveau_vm_unmap(&nvbo->vma);
 		nouveau_vm_put(&nvbo->vma);
+	}
 	kfree(nvbo);
 }