drm/i915: there's no PIPESTAT on HAS_PCH_SPLIT platforms

So don't read it when capturing the error state. This solves
"unclaimed register" messages on Haswell when we have a GPU hang.

V2: Check for HAS_PCH_SPLIT instead of Gen5+ because VLV still has
this register.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -1387,6 +1387,7 @@ static void i915_capture_error_state(struct drm_device *dev)
 	else if (INTEL_INFO(dev)->gen == 6)
 		error->forcewake = I915_READ(FORCEWAKE);
 
+	if (!HAS_PCH_SPLIT(dev))
 		for_each_pipe(pipe)
 			error->pipestat[pipe] = I915_READ(PIPESTAT(pipe));