If there are no displays attached, there is no reason to disable
mclk switching.

Fixes mclks getting set to high when there are no displays attached.
Reviewed-by: NEric Huang <JinhuiEric.Huang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org