radv: clean up setting partial_es_wave for distributed tess on VI

Only needed when the pipeline actually uses tessellation. I don't
think that changes anything, except improving readability.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit d9d14346)