Move it into src/gallium/include/ to make it absolutely clear this is a Mesa detail.
While we're at it, clean up its include sites, including some places
where we can just include kopper_interface.h instead since it includes
mesa_interface.h as its first act.
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28378>