drm/amd/display: Refactor LTTPR cap retrieval

[WHY]
Split LTTPR mode selection between platform support and downstream link support

Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Acked-by: Stylon Wang <stylon.wang@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>