drm/amd/powerplay/vega10: allow mclk switching with no displays

commit 5825acf5 upstream.

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: Eric Huang <JinhuiEric.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>