drm/amd/powerplay/smu7: disable mclk switching for high refresh rates

Even if the vblank period would allow it, it still seems to
be problematic on some cards.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=96868



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