video: mdss: Add DT property to select idle FPS

 * Some display hardware doesn't like going all the way down to 30 FPS
   during idle mode, but work fine for other cases like video. Add a
   DT flag to allow using a higher FPS for idle to save power in
   camera+video cases.

Change-Id: I02c2b67db9609677b7b306d924cebcede0f14e56