drm/amd/display: remove checking sink in is_timing_changed

[Why]
Sometimes, such as sleep wake, the link->local sink pointer changed,
but the dc_stream_state->sink pointer is not changed. The checking
of timing_changed reports wrong result, lead to link tear down
unexpected wrongly.

[How]
SST compare local sink, MST compare proper remote link.

Signed-off-by: Calvin Hou <Calvin.Hou@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>