drm/amd/display: add protection in link encoder matching logic

[Why]
Link encoders are created based on its engine ID. The position of a link
encoder in an array could be null since it didn't be allocated.  Current
matching logic didn't consider this situation and could get null
encoder.

[How]
To add null encoder check to make the matching logic can go to next to
get a valid one.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>