drm/amd/display: Remove NULL assignment for stream_status

[Description]
No need to assign stream_status NULL because it is always
re-assigned before usage. This change is to fix coverity
errors.

Reviewed-by: Nicholas Choi <nicholas.choi@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>