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Combine all displays requested to be enabled into 1 vector to the Screen Manager.
When atomic modeset is enabled, this will allow the screen manager to maintain their combined state and modeset them together.

BUG=1082882
TEST=no changes enabling, ozone_unittests.

Change-Id: Ib8bf31b13f6877c6523aada5c56e5a37f2b64fe7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316155
Commit-Queue: Mark Yacoub <markyacoub@google.com>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800893}