drm/amd/display: isolate 8b 10b link training sequence into its own function

[how]
1. move 8b 10 link trianing into its own function
2. make link status check after a link success link as part of
dp transition to video idle sequence.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>