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

commit d03874c881a049a50e12f285077ab1f9fc2686e1 upstream.

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

Change-Id: If895f63d4694a35acf25005e0ccf745b9633952c
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>