power: few tweaks on Interactive CPU govenor

Change-Id: Ie90b75d3b3be6c98dac0f3acec85d820e3c1e7a7
prop: use Double buffering for FB instead of default Triple buffering

* We got lots of excessive delay (4K+ ms around) reading VSYNC with
  Triple buffering when CPU gets a bit busy, for example, doing some
  interactive tasks.

* Double buffering will alleviate the CPU/GPU's pain then the system
  should go faster.

Change-Id: I8ca4687b07ee242c6f83b43db663ae8a14ff7d1d