drm/nouveau: support init table i2c device identifier 0x81

It appears to be meant to reference the second "default index".
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bios.c

@@ -810,6 +810,9 @@ init_i2c_device_find(struct drm_device *dev, int i2c_index)
 	}
 	if (i2c_index == 0x80)	/* g80+ */
 		i2c_index = dcb->i2c_default_indices & 0xf;
+	else
+	if (i2c_index == 0x81)
+		i2c_index = (dcb->i2c_default_indices & 0xf0) >> 4;
 
 	if (i2c_index > DCB_MAX_NUM_I2C_ENTRIES) {
 		NV_ERROR(dev, "invalid i2c_index 0x%x\n", i2c_index);