drm/amd/display: Cleanup code that enables freesync

This is a follow-up patch to:

Bhawanpreet Lakha    Atomic freesync ASSERT fix

Changes:
- Combine to use one iterator
- Use new DRM iterators.

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