turnip: make the struct slot_value of queries get 2 values

turnip: make the struct slot_value of queries get 2 values

In case of transform feedback query, it writes two integer values,
which one is for primitives written and another is for primitives
generated.

To handle this, the second member of the struct slot_value is worth
to be presented not as a padding.

In addition, we also need to modify get/copy_result to access both
values.

This patch is the prep work for the transform feedback query support.

Tested with
dEQP-VK.pipeline.timestamp.*
dEQP-VK.query_pool.occlusion_query.*

Signed-off-by: Hyunjun Ko <zzoon@igalia.com>
Reviewed-by: Brian Ho <brian@brkho.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4604>