drm/radeon: prep for r6xx/r7xx support

- add r6xx/r7xx regs and macros
- add r6xx/r7xx chip families
- fix register access for regs with offsets >= 0x10000

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>