drm/radeon/kms/atom: bios scratch reg handling updates

- Add missing DFP6 connection state handling
- crtc routing bits not used on DCE4+

Noticed by sylware on phoronix.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>