drm/i915: Only bind to function 0 of the PCI device

Early chipsets (gen2/3) used function 1 as a placeholder for multi-head.
We used to ignore these since they were not assigned to
PCI_CLASS_DISPLAY_VGA. However with 934f992c7 we attempt to bind to all
Intel PCI_CLASS_DISPLAY devices (and functions) to work in multi-gpu
systems. This fails hard on gen2/3.

Reported-by: Ferenc Wágner <wferi@niif.hu>
Tested-by: Ferenc Wágner <wferi@niif.hu>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=28012
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org

drivers/gpu/drm/i915/i915_drv.c

@@ -568,6 +568,14 @@ int i915_reset(struct drm_device *dev, u8 flags)
 static int __devinit
 i915_pci_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 {
+	/* Only bind to function 0 of the device. Early generations
+	 * used function 1 as a placeholder for multi-head. This causes
+	 * us confusion instead, especially on the systems where both
+	 * functions have the same PCI-ID!
+	 */
+	if (PCI_FUNC(pdev->devfn))
+		return -ENODEV;
+
 	return drm_get_pci_dev(pdev, ent, &driver);
 }