drm/amd/display: retain/release at proper places in link_enc assignment

[why]
Need to keep track of number of
references to stream pointer.

[how]
Call stream retain/release whenever
necessary in link_enc table assignment
sequence.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>