Add a null-check for mScreenBrightnessRampAnimator when dumping

This change makes mScreenBrightnessRampAnimator be consistent with all
the other members of DisplayPowerController that are created on
initialize().

Bug: 111900798
Test: dumping DisplayPowerController no longer brings system_server when
      called before DisplayPowerController.initialize()

Change-Id: I91200f8bb84c93e14effa93bdfdf9ded6df7c830