drm/amdgpu/cgs: add refresh rate checking to non-DC display code

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: Eric Huang <JinhuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>