panvk: Fix per-instance attribute handling

panvk: Fix per-instance attribute handling

We were assuming per-vertex attributes so far. Let's extend the logic
to support per-instance attributes with or without custom instance
divisors.  Now that we've got it all hooked up, we can enable
VK_EXT_vertex_attribute_divisor.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15295>