drm/radeon: clarify and extend wb setup on APUs and NI+ asics

Use family rather than DCE check for clarity, also always use
wb on APUs, there will never be AGP variants.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>