Re-enable default display when switching to VR mode.

Previously, in an attempt to minimize the number of changes that got
reset during VR mode transitions, the primary display was not turned back
on.

Bug: 62215749
Test: Switch device into VR mode, observe primary display invalidations.

Change-Id: Idd883e1522f519a0db06fdd88b071007487b6e70
(cherry picked from commit 933ed5ce)