intel: Clean up leak of driver context structure on context destroy.

(cherry picked from commit ddef7dc87b)

src/mesa/drivers/dri/intel/intel_context.c

@@ -804,6 +804,9 @@ intelDestroyContext(__DRIcontextPrivate * driContextPriv)
 
       /* free the Mesa context */
       _mesa_free_context_data(&intel->ctx);
+
+      FREE(intel);
+      driContextPriv->driverPrivate = NULL;
    }
 }