drm/nv50: flush bar1 vm / dma object setup before poking 0x1708

Should fix issues noticed on NVAC (MacBook Pro / ION) since gpuobj
rework.

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

drivers/gpu/drm/nouveau/nv50_instmem.c

@@ -226,6 +226,8 @@ nv50_instmem_init(struct drm_device *dev)
 	nv_wo32(priv->fb_bar, 0x10, 0x00000000);
 	nv_wo32(priv->fb_bar, 0x14, 0x00000000);
 
+	dev_priv->engine.instmem.flush(dev);
+
 	nv_wr32(dev, 0x001708, 0x80000000 | (priv->fb_bar->cinst >> 4));
 	for (i = 0; i < 8; i++)
 		nv_wr32(dev, 0x1900 + (i*4), 0);