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When multiples displays are removed at the same time, commit a disable
modeset of all them together instead of each separately.
This should prevent unplugged MST connectors from being in zombie mode.

BUG=b/172069469, b/169386252
TEST=unplug and replug 2 connectors on MST hub, ScreenManagerTest.CheckMultipleControllersAfterBeingRemoved

Change-Id: If24f9016462b9c69633148202641f7ba6bae9114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512550
Commit-Queue: Mark Yacoub <markyacoub@google.com>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823623}