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."