drm/radeon/kms: add support for router objects

router objects are found on systems that use a mux to control
ddc line to connector routing or to control the actual clock and data
routing from the chip to the connectors.  This patch implements ddc line
routing.

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