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

drivers/gpu/drm/radeon/radeon_pm.c

@@ -852,6 +852,12 @@ static struct radeon_ps *radeon_dpm_pick_power_state(struct radeon_device *rdev,
 			single_display = false;
 	}
 
+	/* 120hz tends to be problematic even if they are under the
+	 * vblank limit.
+	 */
+	if (single_display && (r600_dpm_get_vrefresh(rdev) >= 120))
+		single_display = false;
+
 	/* certain older asics have a separare 3D performance state,
 	 * so try that first if the user selected performance
 	 */