zink: redo slot mapping again for the last time really I mean it

zink: redo slot mapping again for the last time really I mean it

now that shader compiling is happening all at once, we can store the slot
map on zink_gfx_program directly and reserve it dynamically in order to
use up only the slots that are actually being used across all shader stages

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7100>