drm/radeon: 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://bugs.freedesktop.org/show_bug.cgi?id=61115
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/552da6d85a82092146af7b0693595fa2a9ea376b.1452525860.git.lukas@wunner.de

drivers/gpu/drm/radeon/radeon_drv.c

@@ -34,9 +34,11 @@
 #include "radeon_drv.h"
 
 #include <drm/drm_pciids.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_gem.h>
 
@@ -319,6 +321,15 @@ static int radeon_pci_probe(struct pci_dev *pdev,
 {
 	int ret;
 
+	/*
+	 * 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;
+
 	/* Get rid of things like offb */
 	ret = radeon_kick_out_firmware_fb(pdev);
 	if (ret)