drm/amd/display: consider channel coding in configure lttpr mode

[why]
Some lttpr configuration steps are exclusive to 8b/10b channel
coding mode. We need to take channel conding into account.

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>
Acked-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>