intel: add a command buffer scratch bo

It is owned by intel_dev so that all command buffers can share it.  Will be
used for workarounds.
diff --git a/icd/intel/dev.h b/icd/intel/dev.h
index 30fb365..d91b3af 100644
--- a/icd/intel/dev.h
+++ b/icd/intel/dev.h
@@ -56,6 +56,8 @@
 
     struct intel_gpu *gpu;
     struct intel_winsys *winsys;
+    struct intel_bo *cmd_scratch_bo;
+
     struct intel_queue *queues[INTEL_GPU_ENGINE_COUNT];
 };