xf86 drivers need to create RandR object in the PreInit stage,
before the ScreenRec is allocated. Changing the RandR DIX code
to permit this required the addition of functions that later associate the
objects with the related screen.

An additional change is that modes are now global, and no longer associated
with a specific screen. This change actually makes mode management cleaner
as there is no more per-screen list of modes to deal with.

This changes the RandR 1.2 ABI/API for drivers.