display: Don't enable pixel repeat for YUV buffers

 * This can cause horrible things to happen on msm8660 like clock rates
   going to a zillion hz, hard lockups, artifacts, and the smell of
   fresh ozone in the air. Don't do it.

Change-Id: Ic3b8541fc149c593e46ca5ed0c1ee14cab6af71a
hwc: Only perform the yuv+composition workaround on MDP < 4.2

Change-Id: Ie69376b282b9eda58b0e8e010ead14b31367c1a0
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