drm-tests: Add support for EXT_image_flush_external_extension

The null_platform_test needs to call EGLImageFlushExternalEXT before every
page flip if this extension is supported by the driver.

BUG=chromium:656786
CQ-DEPEND=CL:400846
TEST=run test on nyan_big

Change-Id: I133e175fec2193fcc0d85861204c8e64f52521b8
Reviewed-on: https://chromium-review.googlesource.com/399958
Commit-Ready: Haixia Shi <hshi@chromium.org>
Tested-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Haixia Shi <hshi@chromium.org>
3 files changed