This is the expected behavior for multiple monitors on Linux.  mclk switching
must happen in the monitors' blanking period.  Since they likely don't align,
especially if the monitors have different timing, we have to use a fixed mclk. 
The DC modesetting code can lock the timing of multiple monitors if they are
using the exact same timing so that the blanking periods align, but I don't
think the Linux power management code takes this into account at the moment.