intel: compute max_threads in intel_pipeline_shader

This avoids repeated compuations in command buffer building and enables us to
compute the total scratch space when we add support for it.
diff --git a/icd/intel/pipeline.h b/icd/intel/pipeline.h
index 1e04625..8b950d3 100644
--- a/icd/intel/pipeline.h
+++ b/icd/intel/pipeline.h
@@ -120,7 +120,9 @@
 
     XGL_FLAGS barycentric_interps;
 
+    /* these are set up by the driver */
     struct intel_pipeline_rmap *rmap;
+    XGL_UINT max_threads;
 };
 
 /*