[Ozone-DRM] Remove support for overlays on DRM legacy

DRM legacy is never used with overlays due to a lack of synchronization
during page flip. Remove the complexity behing trying to page flip
overlays on legacy. Also move tests that deal with overlays to use
atomic configuration.

BUG=822853
TEST=Ran unittests
TEST=Ran chrome on Samus to validate legacy path still works
TEST=Ran chrome on Eve to ensure atomic path hasn't regressed

Change-Id: Ibc82aafd367c8d12106d3a0e835a3b4b38e0161a
Reviewed-on: https://chromium-review.googlesource.com/1120692
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577610}
11 files changed