drm/amd/display: Enable Freesync over PCon

drm/amd/display: Enable Freesync over PCon

[why]
Enable Freesync over PCon on Linux environment.

[how]
Adding Freesync over PCon support in amdgpu_dm
- Read DPCD for Freesync over PCon capabilitiy
- Add whitelist for compatible branch devices

Reviewed-by: Chao-kai Wang <Stylon.Wang@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>