i965/vue: Make assign_vue_map() take an explicit slot.

i965/vue: Make assign_vue_map() take an explicit slot.

Our plan of assigning consecutive slots doesn't work properly for
separate shader objects - at least, if we want to avoid recompiling them
whenever the interface changes.

As a first step, make assign_vue_map take an explicit slot parameter,
rather than implicitly incrementing it.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>