meson: Move gallium state tracker auto-detection later.
I could move only vl, but I think this is more consistent.
The va autodetection used dep_libdrm, which was not defined at that point in the file. By movig it later we can use dep_libdrm.
AFAICT none of the determined variables is used in between.
Fixes: 392c6092 "vl: Enable DRM by default."