Correct; ddavenport@ do you know a way to detect that overlays are
not supported? Something similar to the is_drm_atomic_supported() [1],
running ioctls or similar?

[1] https://cs.corp.google.com/chromeos_public/src/third_party/autotest/files/client/cros/graphics/graphics_utils.py?q=graphics_utils+package:%5Echromeos_public$&dr&l=1253

Perhaps we could enumerate the planes enumerated in the output of modetest -p. If there are no Overlay planes, then perhaps this could be used as a heuristic to skip the test.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/82aaf153ac2623ea654124a5a9fa34b08262e4e0

commit 82aaf153ac2623ea654124a5a9fa34b08262e4e0
Author: Miguel Casas <mcasas@chromium.org>
Date: Fri Sep 28 02:44:45 2018

autotest: count modetest planes and use it in graphics_HwOverlay

This CL:
- Adds two new methods to graphics_utils: get_modetest_planes()
 and get_max_num_available_drm_planes(). The latter uses the former
 to return the maximum number of overlay planes available in the
 system.
- Teaches graphics_HwOverlays.py to skip the test if there's
 just two or less overlay planes, since in that case we don't
 have any overlays available to promote a DrawQuad to.

Change-Id: I87abe951387d98567a21a9e95355aeaee1899bc0
Bug:  880988 
Reviewed-on: https://chromium-review.googlesource.com/1222491
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Drew Davenport <ddavenport@chromium.org>

[modify] https://crrev.com/82aaf153ac2623ea654124a5a9fa34b08262e4e0/client/site_tests/graphics_HwOverlays/graphics_HwOverlays.py
[modify] https://crrev.com/82aaf153ac2623ea654124a5a9fa34b08262e4e0/client/cros/graphics/graphics_utils.py

These tests are no longer failing. Thanks!

\o/