drm/radeon: disable mclk switching with 120hz+ monitors

These tend to be problematic even if the vblank period is
long enough.  This needs more investigation across a wider
range of displays.  Disable for now.

bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=87796
https://bugs.freedesktop.org/show_bug.cgi?id=89198



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org