drm/amd/display: Call dc_stream_release for remove link enc assignment

[Why]
A porting error resulted in the stream assignment for the link
being retained without being released - a memory leak.

[How]
Fix the porting error by adding back the dc_stream_release() intended
as part of the original patch.

Fixes: 0bb24555 ("drm/amd/display: retain/release at proper places in link_enc assignment")

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>