xf86-video-ati: update to 7.6.1

Glamor support needs work in the xorg recipe as well as testing, it
produced weird artefacts in the picture on my HD6450 when I enabled it a
few months ago. Anyway, glamor wasn't enabled in 7.2.0 so no actual
change.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>