xfree86: Bump ABI_VIDEODRV_VERSION to 25.2

This gives out of tree drivers a fighting chance to build against both
sides of
xorg/xserver!468 .

Reviewed-by: Dave Airlie <airlied@redhat.com>
4 jobs for ABI_VIDEODRV_VERSION-25.2 in 3 minutes and 42 seconds (queued for 1 second)