nir/schedule: Store a pointer to the scoreboard in nir_deps_state

nir/schedule: Store a pointer to the scoreboard in nir_deps_state

nir_deps_state is a struct used as a closure for calculating the
dependencies. Previously it had two fields copied out of the scoreboard.
The closure is initialised in two seperate places. In order to make it
easier to access other members of the scoreboard in the callbacks in
later patches, the closure now just contains a pointer to the scoreboard
and the two bits of copied information are removed.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5561>