mtlib/platform.py: Detect xorg.conf files in ozone/freon world

This was breaking platform_GesturesRegressionTests autotest.

BUG=chromium:452991
TEST="test_that <DUT IP> platform_GesturesRegressionTests" passed on
both link (ozone/freon) and lumpy (X11).

Change-Id: I544155330c60377615588199a50a991c862efd1c
Reviewed-on: https://chromium-review.googlesource.com/243961
Reviewed-by: Dennis Kempin <denniskempin@chromium.org>
Commit-Queue: Andrew de los Reyes <adlr@chromium.org>
Tested-by: Andrew de los Reyes <adlr@chromium.org>
1 file changed