drm/nv4c/bios: disallow retrieving from prom on nv4x igp's

Suggested-by: Marcin Kościelnicki <koriakin@0x04.net>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/subdev/bios/base.c

@@ -130,6 +130,10 @@ nouveau_bios_shadow_prom(struct nouveau_bios *bios)
 	u16 pcir;
 	int i;
 
+	/* there is no prom on nv4x IGP's */
+	if (device->card_type == NV_40 && device->chipset >= 0x4c)
+		return;
+
 	/* enable access to rom */
 	if (device->card_type >= NV_50)
 		pcireg = 0x088050;