drm/nv50: use correct PRAMIN flush register on original nv50

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nouveau_state.c b/drivers/gpu/drm/nouveau/nouveau_state.c
index c58ff9c..0bf79bf 100644
--- a/drivers/gpu/drm/nouveau/nouveau_state.c
+++ b/drivers/gpu/drm/nouveau/nouveau_state.c
@@ -266,7 +266,10 @@
 		engine->instmem.clear		= nv50_instmem_clear;
 		engine->instmem.bind		= nv50_instmem_bind;
 		engine->instmem.unbind		= nv50_instmem_unbind;
-		engine->instmem.flush		= nv50_instmem_flush;
+		if (dev_priv->chipset == 0x50)
+			engine->instmem.flush	= nv50_instmem_flush;
+		else
+			engine->instmem.flush	= nv84_instmem_flush;
 		engine->mc.init			= nv50_mc_init;
 		engine->mc.takedown		= nv50_mc_takedown;
 		engine->timer.init		= nv04_timer_init;