drm/radeon/kms: rework encoder handling

On most newer asics, digital encoders have two links each
and they can be used independantly.  As such, treat them as
separate encoders otherwise the individual links will not
get programmed properly at modeset time.

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