minigbm: Fix consistency in return values

minigbm functions don't use errno; however drmIoctl does. So we need
to return -errno instead of returning exactly what drmIoctl returns.

BUG=none
TEST=builds

Change-Id: I20e00141782ac1407133ee72259fe43381954d26
Reviewed-on: https://chromium-review.googlesource.com/1534878
Commit-Ready: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
9 files changed