Clamp the vblank period to 0 if the refresh rate is larger than
120 hz for non-DC.  This allows us to remove the refresh rate
checks from powerplay for mclk switching.
Reviewed-by: NEric Huang <JinhuiEric.Huang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>