drm/amd/display: add exit training mode and update channel coding in LT

[why]
As recommended by DP specs, source needs to make sure
DPRX exits previous LT mode before configuring new LT params
Nofity what channel coding mode we will use for current link training.

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>