drm/amd/display: rename perform_link_training_int function

[why]
The function's name doesn't represent what it actaully does.
The function implements necessary steps for our hardware to
transition from link training mode back to video idle mode.
Therefore, rename the function as dp_transition_to_video_idle
so everyone can understand it.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>