st/vega: Use vgapi.

Rename vgFooBar to vegaFooBar and use vgapi as the dispatcher.  This
makes sure there is always a current context when the internal functions
are called.  And eglGetProcAddress is finally supported.
diff --git a/configure.ac b/configure.ac
index 4f37e16..dd5581a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1315,6 +1315,9 @@
             # mesa/es is required to build es state tracker
             CORE_DIRS="$CORE_DIRS mesa/es"
             ;;
+        vega)
+            CORE_DIRS="$CORE_DIRS mapi/vgapi"
+            ;;
         esac
     done
     GALLIUM_STATE_TRACKERS_DIRS="$state_trackers"