pipe-loader: Fix memory leak v2

v2:
  - Change driver_name to char*

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
CC: "10.2" <mesa-stable@lists.freedesktop.org>

src/gallium/auxiliary/pipe-loader/pipe_loader_drm.c

@@ -256,7 +256,7 @@ pipe_loader_drm_release(struct pipe_loader_device **dev)
       util_dl_close(ddev->lib);
 
    close(ddev->fd);
-   /* XXX: Free ddev->base.driver_name - strdup at loader_get_driver_for_fd */
+   FREE(ddev->base.driver_name);
    FREE(ddev);
    *dev = NULL;
 }