drm/i915/guc: Replace WARN with a DRM_ERROR

Replace the WARN with a simple if() + error message to squech the sparse
warning that entire wait_for() macro was being stringified:

drivers/gpu/drm/i915/intel_guc_submission.c:658:9: error: too long token expansion
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190408091728.20207-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_guc_submission.c

@@ -650,9 +650,10 @@ static void wait_for_guc_preempt_report(struct intel_engine_cs *engine)
 	struct guc_ctx_report *report =
 		&data->preempt_ctx_report[engine->guc_id];
 
-	WARN_ON(wait_for_atomic(report->report_return_status ==
-				INTEL_GUC_REPORT_STATUS_COMPLETE,
-				GUC_PREEMPT_POSTPROCESS_DELAY_MS));
+	if (wait_for_atomic(report->report_return_status ==
+			    INTEL_GUC_REPORT_STATUS_COMPLETE,
+			    GUC_PREEMPT_POSTPROCESS_DELAY_MS))
+		DRM_ERROR("Timed out waiting for GuC preemption report\n");
 	/*
 	 * GuC is expecting that we're also going to clear the affected context
 	 * counter, let's also reset the return status to not depend on GuC