I have noticed the same issue. It appears DKMS compilation of the kernel module is not working, I have not had the time to investigate it unfortunately. I hope AMD can fix this soon.
use auto logon, ok its not safe that way to use centos 7.3 but there is a bug in the GDM greeter , i fixed by this way.
install the driver, not reboot yet go typ in Terminal = as ROOT
and add these 2 lines
# GDM configuration storage
AutomaticLogin=david <----- log on name !!!
Now reboot and hopefully you have go !! AMD Video would work full now.
rpm -qa | grep amdgpu-pro
FirePro and CentOS is obviously a difficult combination ...
4 days trial and error is enough for me ...
Nvidia card is ordered ..