drm/nouveau: Disable PROM access on init.

On older cards (<nv17) scanout gets blocked when the ROM is being
accessed. PROM access usually comes out enabled from suspend, switch
it off.

Signed-off-by: Francisco Jerez <[e-mail]>
Signed-off-by: Ben Skeggs <[e-mail]>

drivers/gpu/drm/nouveau/nv04_mc.c

···
 	 */
 
 	nv_wr32(dev, NV03_PMC_ENABLE, 0xFFFFFFFF);
+
+	/* Disable PROM access. */
+	nv_wr32(dev, NV_PBUS_PCI_NV_20, NV_PBUS_PCI_NV_20_ROM_SHADOW_ENABLED);
+
 	return 0;
 }