drm/amd/display: Update link training fallback behaviour.

[Why]
Some displays may need several link training attempts before
link training succeeds.

[How]
If training succeeds after falling back to lower link bandwidth,
retry at original link bandwidth instead of abandoning link training
whenever link bandwidth is less than stream bandwidth.

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