xf86-video-ati: Update to 7.5.0

This update brings several bug fixes. Furthermore, it improves Glamor
support, though this feature isn't supported by FreeBSD yet.

Differential Revision: https://reviews.freebsd.org/D1106
Tested by: Many on freebsd-x11@
Reviewed by: bapt@, kwm@
Approved by: bapt@