drm/amd/display: add function to convert hw to dpcd lane settings

[why]
Unify the code which handles the conversion between hw lane setting
and dpcd lane setting.

v2: squash in unused variable fixes (Alex)

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