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/cmd.h b/icd/intel/cmd.h
index 74e1875..16049b9 100644
--- a/icd/intel/cmd.h
+++ b/icd/intel/cmd.h
@@ -99,6 +99,7 @@
     struct intel_obj obj;
 
     struct intel_dev *dev;
+    struct intel_bo *scratch_bo;
     int pipeline_select;
 
     struct intel_cmd_reloc *relocs;