overlay: Bring back debug.mdpcomp.maxlayer

 * We're having underrun issues with 4-layer composition. It generally
   works, but when the system CPU ramps down and the device is mostly
   idle, underrun is happening.
 * Limiting to 3 layers solves the problem temporarily on these devices.
 * Readd the option so that the issue can be debugged without afflicting
   our current users with massive amounts of flickering.

Change-Id: I7ab7d013f65eaf12a3d21a91229dcaad6a815069