msm8974-common: Move NUM_FB_BUFFERS to msm8974-common

Change-Id: I1d5007477277445b5abf8bafb7fcfd2d7c2c0f39
IR: Increase Buffer size

  Fixes JIRA CYAN-5239

Change-Id: I6d6a2cd48f9b7b485099c84517cab9e7b08d1e3a