zink: do not use reservations for stream-out

reservations are accumulated for all shader-stages in a program without
resetting it. But stream-out is completely orthogonal to all other
inputs and outputs, so they don't matter for this stuff at all.

So let's drop considering reservations here, and simply count how many
generic outputs we have here instead.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7986>

src/gallium/drivers/zink/nir_to_spirv/nir_to_spirv.c

@@ -969,7 +969,7 @@ emit_so_info(struct ntv_context *ctx, const struct zink_so_info *so_info)
        * so we need to ensure that the new xfb location slot doesn't conflict with any previously-emitted
        * outputs.
        */
-      uint32_t location = reserve_slot(ctx);
+      uint32_t location = ctx->shader_slots_reserved + i;
       assert(location < VARYING_SLOT_VAR0);
       spirv_builder_emit_location(&ctx->builder, var_id, location);