drm/radeon/atom: select the proper number of lanes in transmitter setup

We need to check for DVI vs. HDMI when setting up duallink since
HDMI is single link only.  Fixes 4k modes on newer asics.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=75223

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