radeonsi: flush TC L2 cache for indirect draw data

This fixes a bug when indirect draw data is generated by transform
feedback.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 2852dedaa0c45e426a53ba0042ecdb0f1b87950f)