etnaviv: nir: call nir_remove_dead_variables(..) before linking setup

etnaviv: nir: call nir_remove_dead_variables(..) before linking setup

There are cases where there is a chain to an unused nir variable that get removed
by nir_opt_dce. This breaks our current linker as the variable can still be accessed
via nir_foreach_shader_in_variable(..) macro.

So lets call nir_remove_dead_variables(..) just before we setup our linking.

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23673>