mb/lenovo: Support dual graphics for xx20/xx30 ThinkPads

Add CMOS option that allows to use both integrated and discrete GPU.

Tested on ThinkPad W530.

Change-Id: I8842fef0fa1235eb91abf6b7e655ed4d8598adc7
Signed-off-by: Evgeny Zinoviev <me@ch1p.com>
Reviewed-on: https://review.coreboot.org/28393
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/mainboard/lenovo/t420s/romstage.c b/src/mainboard/lenovo/t420s/romstage.c
index 55011cf2..be8052c 100644
--- a/src/mainboard/lenovo/t420s/romstage.c
+++ b/src/mainboard/lenovo/t420s/romstage.c
@@ -32,6 +32,9 @@
 
 	early_hybrid_graphics(&igd, &peg);
 
+	if (peg && igd)
+		return;
+
 	/* Hide disabled devices */
 	reg32 = pci_read_config32(PCI_DEV(0, 0, 0), DEVEN);
 	reg32 &= ~(DEVEN_PEG10 | DEVEN_IGD);