gallivm: change coordinate handling throughout functions

gallivm: change coordinate handling throughout functions

Instead of passing s,t,r coordinates pass a coord array - the reason is that
I need to pass more coords (in particular for shadow "coord", future will also
need another one for cube map arrays) so just pass them as an array.
Also, to simplify things, use fixed location for the shadow reference value I
want to get rid of the silly "where is the right coord value" game.
Keep old-style however for aos sampling (which is not going to need shadow
coord, though for cube map arrays it still would need fixing).
(Next patch will pass those through using the new arrangement directly from
sampler interface.)

v2: fix up soa split path (unreachable currently but still...)

Reviewed-by: Zack Rusin <zackr@vmware.com>