drm/i915: add _DSM support

The _DSM method on the integrated graphics device can tell us which
connectors are muxable, so add support for making the call and parsing
out the connector info.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: fix compiler warnings for using uninitialized 'result' and
downgrade error message for non-switchable devices]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 73ad8bf..e4ffcd3 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1136,6 +1136,15 @@
 static inline void intel_opregion_enable_asle(struct drm_device *dev) { return; }
 #endif
 
+/* intel_acpi.c */
+#ifdef CONFIG_ACPI
+extern void intel_register_dsm_handler(void);
+extern void intel_unregister_dsm_handler(void);
+#else
+static inline void intel_register_dsm_handler(void) { return; }
+static inline void intel_unregister_dsm_handler(void) { return; }
+#endif /* CONFIG_ACPI */
+
 /* modesetting */
 extern void intel_modeset_init(struct drm_device *dev);
 extern void intel_modeset_cleanup(struct drm_device *dev);