drm/radeon/kms: get lvds info for DIG LVTMA and UNIPHY encoders

Noticed by Rafał Miłecki on dri-devel.  On r6xx/r7xx hardware, laptop
panels can be driven by KLDSCP_LVTMA or UNIPHY.

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