hwc: Add support for perf display hint

- Set the perf hint when there is only one updating layer, which helps
  in reducing cpu params and thus reduce power.
- This feature is enabled with the property persist.mdpcomp_perfhint,
  which is set to a positive value, which will be the window of frames
  before the perf hint is set

CRs-fixed: 765460
Change-Id: Ie21e1cd201afab16bedd3c750ea3b0230769969a
2 files changed