drm/i915: Clear out old GT FIFO errors in intel_uncore_early_sanitize()

The BIOS or someone else might have done something bad and there
might be old GT FIFO erros reported in GTFIFODBG. Clear those out
in intel_uncore_early_sanitize() to make sure we don't mistake them
for our problems.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_uncore.c

@@ -332,6 +332,11 @@ void intel_uncore_early_sanitize(struct drm_device *dev)
 		DRM_INFO("Found %zuMB of eLLC\n", dev_priv->ellc_size);
 	}
 
+	/* clear out old GT FIFO errors */
+	if (IS_GEN6(dev) || IS_GEN7(dev))
+		__raw_i915_write32(dev_priv, GTFIFODBG,
+				   __raw_i915_read32(dev_priv, GTFIFODBG));
+
 	intel_uncore_forcewake_reset(dev);
 }