drm/amd/display: Keep DVI_SINGLE_LINK signal if low clk

If user is using DVI->HDMI dongle dual link signal might pose a
problem. Keep single link signal type if clk is lower than
max tmds clk.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>