drm/amd/display: Fix for link encoder access for MST.

[Why]
Link encoder in the link could be null for certain links.

[How]
If link encoder in the link is null then get the link encoder
from the stream.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>