gbm: don't return void

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/gbm/backends/dri/gbm_dri.c

@@ -174,7 +174,7 @@ swrast_put_image(__DRIdrawable *driDrawable,
                  char          *data,
                  void          *loaderPrivate)
 {
-   return swrast_put_image2(driDrawable, op, x, y, width, height,
+   swrast_put_image2(driDrawable, op, x, y, width, height,
                             width * 4, data, loaderPrivate);
 }