tu: Abstract driver-specific const state

tu: Abstract driver-specific const state

Right now, we pass around the push constant state in a lot of places,
but we'll want to add other driver-managed constants. Add a struct which
we can add to, and separate out the total driver-reserved constants from
the size of push constants.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18554>