The Radeon firmware is coded into the kernel in NetBSD/OpenBSD and is being loading.
The BusID PCI:1:5:0 is also correct. J65nko post is the next step.
Alternatively you can look for working xorg.conf for a similar laptop in NetBSD. On a quick search I found this published, working,
xorg.conf for a HP Pavilion laptop with an ati card. It is for OpenBSD but the device naming conventions are the same.