radeon: consolidate asic-specific function decls for r600 & later

Now all the asic specific stuff ist mostly hid in radeon_asic.*

Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>