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: NFrancisco Jerez <currojerez@riseup.net>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv04_mc.c

@@ -11,6 +11,10 @@ nv04_mc_init(struct drm_device *dev)
 	 */
 
 	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;
 }