x11-drivers/xf86-video-ati-legacy: Mark broken

xf86-video-ati-legacy does not currently build with xorg-server 1.20.7.
A patch to fix the build is available here,
https://reviews.freebsd.org/D23817 , but it needs more testing.
Mark this port as broken while the issues are being sorted out, to avoid
confusion and messages from the build cluster.