radv: disable viewport clamping even if FS doesn't write Z

This fixes new CTS dEQP-VK.pipeline.depth_range_unrestricted.*.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 0a313cc2)