drm/radeon/kms: fix DP setup on TRAVIS bridges

Supposedly both NUTMEG and TRAVIS should use the same
panel mode, but switching the panel mode for TRAVIS
gets things working.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=41569



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>