radv: fix VK_EXT_conditional_rendering visibility

It's actually just the opposite.

This fixes the new Sascha conditionalrender demo.

CC: 18.2 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit abdf396c)