drm/amd/display: fix link training regression for 1 or 2 lane

[why]
We have a regression that cause maximize lane settings to use
uninitialized data from unused lanes.
This will cause link training to fail for 1 or 2 lanes because the lane
adjust is populated incorrectly sometimes.

v2: fix build without CONFIG_DRM_AMD_DC_DCN (Alex)

Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>