This is required because, in the radeon driver, we can't left-shift in a
central place, like it was done in the i915 driver.
Reviewed by: kib@, kan@, avg@
Tested by: kib@, avg@
works with new generations of GPUs (IronLake, SandyBridge and
supposedly IvyBridge).
The driver is not connected to the build yet.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week