drm/radeon/kms: add support for external tmds on legacy boards

This enables initialization of external tmds chips on pre-atom
and mac systems.  Macs are untested.  Also, some macs have single
link tmds chips while others have dual link tmds chips.  We need
to figure out which ones have which.

This gets external TMDS working on my RS485 and RV380.

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