drm/radeon/kms: handle DP bridges

Fusion hardware often has DP to VGA/LVDS/TMDS bridges to
handle non-DP encoders.  Internally we treat them mostly
like DP.

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