display: Fix analog display content protection

Content protection would be reported as enabled while mirroring to an
external display with an unsecure (e.g. analog) connection type.

Bug: 929449
Test: display_unittests
Change-Id: I4929ff4fa591616f27eaed01875455c86504e91b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674404
Commit-Queue: Dominik Laskowski <domlaskowski@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680038}
5 files changed