drm/amd/display: fix stale info in link encoder assignment

[Why]
The link encoder assignment leaves the old stream data when it was
unassigned. When the clear encoder assignment is called, it based on the
old stale data to access the de-allocated stream.

[How]
There should be no need to explicitly clean up the link encoder
assignment if the unassign loop does the work properly, the loop should
base on the current state to clean up the assignment.

Also, the unassignment should better clean up the values in the
assignement slots as well.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Roy Chan <roy.chan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>