sde: Add set power mode support for virtual display.

1. Set the power mode for virtual display while setting power mode for
   primary, as surfaceflinger does not invoke SetPowerMode() for
   virtual display.
2. Invoke null commit on each display to release all the pipes connected
   to it on SetPowerMode().

Change-Id: I8992850a7acbfdf8ab0de3216d92cf4f28fbaaeb
6 files changed