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_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index 3bbe723..f451af6 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -1244,6 +1244,8 @@
 	if (ret)
 		goto cleanup_ringbuffer;
 
+	intel_register_dsm_handler();
+
 	ret = vga_switcheroo_register_client(dev->pdev,
 					     i915_switcheroo_set_state,
 					     i915_switcheroo_can_switch);