drm/amd/display: Refactor LTTPR mode selection

[WHY]
Previously, LTTPR mode was decided during detection which makes
link training inflexible as mode can't be dynamically changed.

[HOW]
-Remove lttpr_mode from link struct, and move to link training settings
-Defer choosing LTTPR mode until link training

Other DP changes included:
-Only use fixed vs/pe link training sequence for 8b/10b encoding
-Restrict fixed vs aux timeout workaround to Yellow Carp family

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>