zink: store nir as serialized on zink_shader structs

zink: store nir as serialized on zink_shader structs

nir_shader objects are hefty, and they really add up when there's a lot
of them. there's also not much use in keeping them around, as any time
they'll be used, they're always cloned first, and deserializing isn't
likely to be any slower than a clone

cuts driver memory usage by ~40% for tomb raider

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22266>