drm/i915: Disable stolen memory when DMAR is active

We have reports of heavy screen corruption if we try to use the stolen
memory reserved by the BIOS whilst the DMA-Remapper is active. This
quirk may be only specific to a few machines or BIOSes, but first lets
apply the big hammer and always disable use of stolen memory when DMAR
is active.

v2 by Jani: Rebase on -fixes, only look at intel_iommu_gfx_mapped.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68535
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_gem_stolen.c

@@ -214,6 +214,13 @@ int i915_gem_init_stolen(struct drm_device *dev)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	int bios_reserved = 0;
 
+#ifdef CONFIG_INTEL_IOMMU
+	if (intel_iommu_gfx_mapped) {
+		DRM_INFO("DMAR active, disabling use of stolen memory\n");
+		return 0;
+	}
+#endif
+
 	if (dev_priv->gtt.stolen_size == 0)
 		return 0;