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