drm/amd/display: Don't compare same stream for synchronized vblank

[Why]
When determining synchronzied vblank we don't need to compare the stream
with itself

[How]
If comparing same stream, continue to next iteration

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>