drm/radeon/kms: fix TRAVIS panel setup

Different versions of the DP to LVDS bridge chip
need different panel mode settings depending on
the chip version used.

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

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>