resurrect old port: xf86-video-ati@479538 as xf86-video-ati-legacy

As there are some issues with the later xf86-video-ati revisions on old DRMs
such as drm-legacy-kmod, resurrect the old version of the port as legacy. Add
conflict with newer port. Note intended use in pkg-descr.

Approved by: x11 (zeising)