intel: add intel_handle

intel_handle is the base struct of any API handle, with its own helper
functions.  Move magic from intel_base to the new intel_handle.
diff --git a/icd/intel/gpu.h b/icd/intel/gpu.h
index 5230afc..b82c0b3 100644
--- a/icd/intel/gpu.h
+++ b/icd/intel/gpu.h
@@ -56,8 +56,7 @@
  * intel_gpu is the only object that does not inherit from intel_base.
  */
 struct intel_gpu {
-    /* the loader expects a "void *" at the beginning */
-    void *loader_data;
+    struct intel_handle handle;
 
     struct intel_gpu *next;