drm/amd/display: expose skip_detection_link_training to debugfs

[Why] within dc link detecion, dp link training will be
executed for external sst dp. for debug purpose, we may
need skip dp link training.

[How] expose dc debug option to skip_detection_link_training
to debugfs

Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: hersen wu <hersenxs.wu@amd.com>