drm/amd/display: Fix null pointer dereference for encoders

[Why]
Links which are dynamically assigned link encoders have their link
encoder set to NULL.

[How]
Check that a pointer to a link_encoder object is non-NULL before using
it.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>