drm/amd/display: prevent reading unitialized links

[why/how]
The function can be called on boot or after suspend when
links are not initialized, to prevent it guard it with
NULL pointer check

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>