drm/amd/display: Don't use dc_link in link_encoder

dc_link is at a higher level than link_encoder, and we only want
higher-level components to be able to access lower-level ones,
not the other way around.

Signed-off-by: Andrew Jiang <Andrew.Jiang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>