radeonsi: do VGT_FLUSH when switching NGG -> legacy on Sienna Cichlid

Other chips don't need this.

Fixes: 9538b9a68ed - radeonsi: add support for Sienna Cichlid

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7542>
(cherry picked from commit c4ebdf9ee7eac7241b5185d7d496e96384a41ecb)
2 files changed