drm/radeon/kms: fix handling of DP to LVDS bridges

They need to be treated like eDP rather than DP.

May fix:
https://bugzilla.kernel.org/show_bug.cgi?id=34822



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