icd: Make driver use the same definition for dispatch table as loader uses

Loader and icd were using two different structs to define dispatch table.
Consolidate to to use the dispatch table defined in xglLayer.h so we
will not have issues with them getting out of sync.
diff --git a/icd/README b/icd/README
index 2daa22c..cfa7b38 100644
--- a/icd/README
+++ b/icd/README
@@ -1,6 +1,6 @@
 common/ provides helper and utility functions, as well as all XGL entry points
 except xglInitAndEnumerateGpus.  Hardware drivers are required to provide that
-function, and to embed a "struct icd_dispatch_table *" as the first member of
+function, and to embed a "XGL_LAYER_DISPATCH_TABLE *" as the first member of
 XGL_PHYSICAL_GPU and all XGL_BASE_OBJECT.
 
 Thread safety