drm/i915: Capture ERROR register on Sandybridge hangs

This holds error state from the main graphics arbiter mainly involving
the DMA engine and address translation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 29cbcb3..2a29497 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -592,13 +592,11 @@
 	error->pipeastat = I915_READ(PIPEASTAT);
 	error->pipebstat = I915_READ(PIPEBSTAT);
 	error->instpm = I915_READ(INSTPM);
-	if (INTEL_INFO(dev)->gen < 4) {
-		error->ipeir = I915_READ(IPEIR);
-		error->ipehr = I915_READ(IPEHR);
-		error->instdone = I915_READ(INSTDONE);
-		error->acthd = I915_READ(ACTHD);
-		error->bbaddr = 0;
-	} else {
+	error->error = 0;
+	if (INTEL_INFO(dev)->gen >= 6) {
+		error->error = I915_READ(ERROR_GEN6);
+	}
+	if (INTEL_INFO(dev)->gen >= 4) {
 		error->ipeir = I915_READ(IPEIR_I965);
 		error->ipehr = I915_READ(IPEHR_I965);
 		error->instdone = I915_READ(INSTDONE_I965);
@@ -606,6 +604,12 @@
 		error->instdone1 = I915_READ(INSTDONE1);
 		error->acthd = I915_READ(ACTHD_I965);
 		error->bbaddr = I915_READ64(BB_ADDR);
+	} else {
+		error->ipeir = I915_READ(IPEIR);
+		error->ipehr = I915_READ(IPEHR);
+		error->instdone = I915_READ(INSTDONE);
+		error->acthd = I915_READ(ACTHD);
+		error->bbaddr = 0;
 	}
 
 	bbaddr = i915_ringbuffer_last_batch(dev);