drm/amd/display: Should disable when new stream is null

core_link_disable_stream should be called when the new stream is null
(i.e. want to disable). Modify the if condition to reflect that.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>