drm/radeon/kms: fix DCE4.1 dig routing (v2)

Works more like DCE4.0 despite what the docs say.
This fixes blank screen issues when changing crtc
routing due to incorrect crtc to dig mapping.

v2: only two DIGx blocks, routing is hardcoded
based on link.

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