nb/x4x: Add ramstage IGD disable function

This disables VGA cycles on IGD when an external VGA device is
found. This allows PCI or PCIe devices to be the 'main' VGA device if
found, while the IGD is still available.

TESTED on ga-g41m-es2l: SeaBIOS shows payload on external GPU while
linux (4.10) can use both as a framebuffer simultaneously without any
extra configuration.

Change-Id: I74890918feb0f1ff6b971c4aaa96f1f7b75266ac
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/18504
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
diff --git a/src/northbridge/intel/x4x/gma.c b/src/northbridge/intel/x4x/gma.c
index f4b2b2e..c2df074 100644
--- a/src/northbridge/intel/x4x/gma.c
+++ b/src/northbridge/intel/x4x/gma.c
@@ -376,6 +376,16 @@
 		pci_dev_init(dev);
 }
 
+static void gma_func0_disable(struct device *dev)
+{
+	struct device *dev_host = dev_find_slot(0, PCI_DEVFN(0, 0));
+	u16 ggc;
+
+	ggc = pci_read_config16(dev_host, D0F0_GGC);
+	ggc |= (1 << 1); /* VGA cycles to discrete GPU */
+	pci_write_config16(dev_host, D0F0_GGC, ggc);
+}
+
 static void gma_set_subsystem(device_t dev, unsigned int vendor,
 			unsigned int device)
 {
@@ -419,6 +429,7 @@
 	.acpi_fill_ssdt_generator = gma_ssdt,
 	.init = gma_func0_init,
 	.ops_pci = &gma_pci_ops,
+	.disable = gma_func0_disable,
 };
 
 static const unsigned short pci_device_ids[] = {