1) If you purchased your system with Linux from your OEM (HP,Dell, Lenovo, etc), you go through them.
2) If you acquired the driver through a distribution, start with their bugtracking system (they know the patches and changes they have made, AMD doesn't).
3) If you acquired the driver through amd.com, then http://www.amd.com/us/LinuxCrewSurvey is the formal location for feedback.
AMD does have "ears" in most of the above locations and is generally aware of what is going on. Unfortunately, 1:1 interaction on a formal bug report is not the way AMD operates.
Now for unofficial reports you obviously have forums and so on. Also a number of years ago I asked the forum members to conentrate bug reports on a bugzilla vs a set of "known issues" forums. The result was http://ati.cchtml.com/ and it is my preferred "community" bug aggregation resource. Again, we see all the bug activity on there, if you are lucky and the issue you are posting is relevant to our current focus, AMD employees also post there too.
Bear in mind that in a very complex piece of software, there will always be many bugs. The prioritization and resolution of those bugs is always a fight against new hardware, new features and formal customer engagement. What seems like a critical to bug to one, may be
a low priority issue to another.