drm/amd/display: retain/release stream pointer in link enc table

[why]
At every reference to stream pointer, we need to increment/decrement the
kref_count.  Not doing so will result in invalid stream pointer still
alive after hibernate cycle.

[how]
Call stream retain/release whenever the link encoder assignment is set
to true/false since it indicates if we want to reference the stream
pointer or not.

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: Sung Joon Kim <sungkim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>