I have recently installed the xorg-11-drv-ati-7.7.0 on my iMac 2010. sources can found at.

https://lists.x.org/archives/xorg-announce/2016-April/002684.html

http://CentOS.cms4all.org/CentOS/7/drivers/

http://CentOS.cms4all.org/CentOS/7/drivers/SRPM/xorg-x11-drv-ati-7.7.0-1.el7.CentOS.src.rpm

Don’t forget that i use a newer mesa 11.1.2 and kernel 4.4.8 its. The kernel 3.10 was not build for that „new“ graphic cards“. Only small tests are done, but works pretty much.

Sincerely

Andreas Benzler