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

Ported from RadeonSI.

Cc: 20.2
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 0790105f2f0d1813dc2ecaf5e12e7131730a8bc1)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8014>
1 file changed