drm/amd/display: Enumerate LTTPR modes

[WHY]
There are three possible modes for LTTPR:
- Non-LTTPR mode, where AUX timeout is 400 us and no per-hop link
training is done
- LTTPR Transparent mode, where AUX timeout is 3200 us and no per-hop
link training is done
- LTTPR Non-Transparent mode, where AUX timeout is 3200 us and per-hop
link training is done

[HOW]
Use an enum instead of a bool to track LTTPR state; modify comparisons
accordingly.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>