drm-tests: vgem_test: don't log everything to stderr

On wmatrix the vgem_test shows up failing on every platform because
of this. Only log to stderr in the case of an error.

BUG=chromium:654131
TEST=vgem_test passes on cyan

Change-Id: Iebf885b480a7fd1530424d14fe7a91633067a091
Reviewed-on: https://chromium-review.googlesource.com/450897
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
1 file changed