drm/amd/display: Add 120Hz support for freesync video mode

[Why]
For those video format with 60 fps, the user space player could
ask for 120Hz for playback.

[How]
Add 120 in the table of common rates.

Signed-off-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>