drm/nouveau/kms/nv50-: attach immutable zpos property to planes

Defaulting to the fixed layout enforced in HW by EVO, and that we
currently use by default on NVD.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv50/disp.c

@@ -2316,6 +2316,7 @@ nv50_display_create(struct drm_device *dev)
 	disp->disp = &nouveau_display(dev)->disp;
 	dev->mode_config.funcs = &nv50_disp_func;
 	dev->mode_config.quirk_addfb_prefer_xbgr_30bpp = true;
+	dev->mode_config.normalize_zpos = true;
 
 	/* small shared memory area we use for notifiers and semaphores */
 	ret = nouveau_bo_new(&drm->client, 4096, 0x1000, TTM_PL_FLAG_VRAM,