radv: fix centroid interpolation

It's legal to set the centroid and sample interpolation modes
when MSAA disabled. So, we have to initialize the centroid
inputs because the hardware doesn't.

This fixes rendering issues with DXVK and The Witness, World of
Warcraft, Trackmania and probably more games.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106315
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102390


CC: 18.0 18.1 <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 73df16dc)