drm/i915: disable rc6 on ilk when vt-d is enabled

It blows up. And hopefully this is the root-cause of the mysterious
rc6 related hang on ilk. For reference, the commit that enabled rc6 on
ilk again is:

commit 456470e
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Aug 8 23:35:40 2012 +0200

    drm/i915: enable rc6 on ilk again

Reported-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -2372,6 +2372,11 @@ int intel_enable_rc6(const struct drm_device *dev)
 		return i915_enable_rc6;
 
 	if (INTEL_INFO(dev)->gen == 5) {
+#ifdef CONFIG_INTEL_IOMMU
+		/* Disable rc6 on ilk if VT-d is on. */
+		if (intel_iommu_gfx_mapped)
+			return false;
+#endif
 		DRM_DEBUG_DRIVER("Ironlake: only RC6 available\n");
 		return INTEL_RC6_ENABLE;
 	}