With an RV635 (HD 3650 FireGL mobile) I get corruption on start of X *if* I start system with no radeon loaded and KMS not enabled.
If I let X load the DRM driver when X starts I will see a stripe rainbow corruption ||||||||||||||||||||||||||||| pattern such as mentioned here. The mouse cursor is square.
I do not know when this started but this may have always been the case since some distributions do not load radeon firmware on start (thus there's no KMS).
I can try to git bisect but with no beginning point or end it might take awhile to find assuming it hasn't always done this.
WORKAROUND: Force load radeon with modeset=1 in initscript prior to X starting fixes this.
To use kms, radeon must be loaded before starting X.