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.c b/icd/intel/cmd.c
index a7e6dc7..ebf84a4 100644
--- a/icd/intel/cmd.c
+++ b/icd/intel/cmd.c
@@ -179,6 +179,7 @@
     cmd->obj.destroy = cmd_destroy;
 
     cmd->dev = dev;
+    cmd->scratch_bo = dev->cmd_scratch_bo;
     cmd->pipeline_select = pipeline_select;
 
     /*