radv, ac/nir: Fix multiview layer export for mesh shaders.

radv, ac/nir: Fix multiview layer export for mesh shaders.

Unfortunately, radv_lower_multiview is not suitable for mesh shaders
because it can't know the mapping between API mesh shader
invocations and output primitives.

Additionally, when lowering view id to layer, it must be created
as a per-primitive PS input.

Fixes: d32656bc65a7c0165e53d551e23462d626bbf870
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16504>