drm/amd/display: Fix uninitialized variables in amdgpu_dm_debugfs

[WHAT]
Some fields in struct dc_link_settings and link_training_settings are
not initialized and using them can cause unexpected results.

[HOW]
Initialize struct dc_link_settings and link_training_settings to zero.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>