radv: Enable NV_mesh_shader with a perftest flag.

radv: Enable NV_mesh_shader with a perftest flag.

We don't plan to support NV_mesh_shader officially on RADV,
because it performs poorly on AMD hardware. However, we are
implementing this extension to get some experience with mesh
shader technology.

Users should not rely on this support because we are going
to remove it if/when a potential cross-vendor extension appears.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13580>