drm/i915: Defer probe if gmux is present but its driver isn't

gmux is a microcontroller built into dual GPU MacBook Pros.
On pre-retina MBPs, if we're the inactive GPU, we need apple-gmux
to temporarily switch DDC so that we can probe the panel's EDID.

The checks for CONFIG_VGA_ARB and CONFIG_VGA_SWITCHEROO are necessary
because if either of them is disabled but gmux is present, the driver
would never load, even if we're the active GPU. (vga_default_device()
would evaluate to NULL and vga_switcheroo_handler_flags() would
evaluate to 0.)

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115
Tested-by: Lukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/f56ee6a0600a3e1bb5bed4d0db4ed9ade7445c47.1452525860.git.lukas@wunner.de

drivers/gpu/drm/i915/i915_drv.c

@@ -35,9 +35,12 @@
 #include "i915_trace.h"
 #include "intel_drv.h"
 
+#include <linux/apple-gmux.h>
 #include <linux/console.h>
 #include <linux/module.h>
 #include <linux/pm_runtime.h>
+#include <linux/vgaarb.h>
+#include <linux/vga_switcheroo.h>
 #include <drm/drm_crtc_helper.h>
 
 static struct drm_driver driver;
@@ -969,6 +972,15 @@ static int i915_pci_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 	if (PCI_FUNC(pdev->devfn))
 		return -ENODEV;
 
+	/*
+	 * apple-gmux is needed on dual GPU MacBook Pro
+	 * to probe the panel if we're the inactive GPU.
+	 */
+	if (IS_ENABLED(CONFIG_VGA_ARB) && IS_ENABLED(CONFIG_VGA_SWITCHEROO) &&
+	    apple_gmux_present() && pdev != vga_default_device() &&
+	    !vga_switcheroo_handler_flags())
+		return -EPROBE_DEFER;
+
 	return drm_get_pci_dev(pdev, ent, &driver);
 }