drm/amd/display: Fix issue with link_active state not correct for MST

[Why]
For MST, link not disabled until all streams disabled

[How]
Add check for stream_count before setting link_active = false for MST

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>