First step of Radeon DRI unification:

First step of Radeon DRI unification:
- Makes all three drivers use the same screen structure and setup code, with a
  few ifdefs for the separate compilation to deal with symbols not being
  available to all drivers and the fact that we have no mechanism for dealing
  with different config options for different chip families in the same driver.
  These issues should be dealt with later.
- Introduces IS_R[123]00_CLASS(radeonScreenPtr) macro for code for taking
  different paths depending on the general class of chipset.
- Adds many new R300-class PCI IDs, though not all those listed in
  radeon_driver.c.