hwc: Replace redundant display_commit with Overlay displayCommit

  1. As part of Writeback class implementation, display commit is
     implemented in Overlay class.
     Replace display_commit function with Overlay::displayCommit
     function.
  2. Fix return value of hwc_blank function

Change-Id: Ie54fa9876495608a60216025e57cf4de7f6cb03c
1 file changed