Comment # 3 on bug 1134475 from Stefan Dirsch

radeondrmfb is the fb (=framebuffer) *implementation* of radeon's DRM (=direct
rendering manager/kernel mode setting) driver/kernel module 'radeon'.
'radeonfb' is an ancient separate framebuffer driver/kernel module for radeon
cards. We never built/shipped, let alone enabled them by default. This should
explains things a bit.

So my conclusion would be, that perhaps radeon driver is not initialized yet
completely when desktop is starting.