drm/amd/display: Fix in dp link-training when updating payload allocation table

[Why & How]
Check if aux is not accessible before updating payload allocation table.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>