drm/radeon/kms: add missing param for dce3.2 DP transmitter setup

This is used during phy init to set up the phy for DP.  This may
fix DP problems on DCE3.2 cards.

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