drm/nouveau/aux: mask off higher bits of auxch index in i2c table entry

At least partially fixes DP output detection on W530.  Not sure if more
issues remain, or if my adaptor is just behaving weirdly (it does that
sometimes).

In any case, this patch is necessary.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_i2c.c

@@ -229,7 +229,7 @@ nouveau_i2c_init(struct drm_device *dev)
 			}
 			break;
 		case 6: /* NV50- DP AUX */
-			port->drive = entry[0];
+			port->drive = entry[0] & 0x0f;
 			port->sense = port->drive;
 			port->adapter.algo = &nouveau_dp_i2c_algo;
 			break;