ALSA: hda - fix use-after-free after module unload

commit ab58d8cc upstream.

register_vga_switcheroo() sets the PM ops from the hda structure which
is freed later in azx_free. Make sure that these ops are cleared.

Caught by KASAN, initially noticed due to a general protection fault.

Fixes: 246efa4a ("snd/hda: add runtime suspend/resume on optimus support (v4)")
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

sound/pci/hda/hda_intel.c

@@ -950,8 +950,10 @@ static int azx_free(struct azx *chip)
 	if (use_vga_switcheroo(chip)) {
 		if (chip->disabled && chip->bus)
 			snd_hda_unlock_devices(chip->bus);
-		if (chip->vga_switcheroo_registered)
+		if (chip->vga_switcheroo_registered) {
 			vga_switcheroo_unregister_client(chip->pci);
+			vga_switcheroo_fini_domain_pm_ops(chip->card->dev);
+		}
 	}
 
 	if (chip->initialized) {