CHROMIUM: move fence array release to a worker

So flush_work is not called from atomic context.

BUG=b:63085243
TEST=run cts  --skip-device-info --module CtsUiHostTestCases --test android.ui.cts.TaskSwitchingTest#testTaskSwitching

Change-Id: I8fc46b96dab9c464c7b83f74a792444f69f63ae2
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/553543
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>