drm/amd/display: Fix a few parts in debugfs entry

[Why & How]
1. To leverage and simplify the igt tool. Change the value of
retrain when calling preferred training settings.
Now, writing value to link settings triggers link training immediately.

2. Add dc lock when triggering link training in edp_ilr_write()

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>