drm/amd/display: Only add stream to freesync when modeset required

This is a follow-up patch to:

Leo (Sunpeng) Li    Cleanup code that enables freesync

We should only add a stream to freesync if a modeset was requested, so
we don't fill the core freesync map with pointless streams.

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>