ozone: drm: Allow overriding GbmDevice & GbmBuffer in tests

Make these polymorphic types so that we can provide a fake implementation
of libgbm for test purposes.

Bug: 869206
Test: ozone_unittests
Change-Id: I773cdac62bade798fbcddac54a2d5557ce1da8ce
Reviewed-on: https://chromium-review.googlesource.com/1162655
Commit-Queue: Michael Spang <[email protected]>
Reviewed-by: Daniel Nicoara <[email protected]>
Cr-Commit-Position: refs/heads/master@{#580669}
15 files changed