minigbm: cleanup exynos backend leftovers

BUG=b:199524294
TEST=CQ

Change-Id: I9a82304247bd506b24e08219fc359874f39fd221
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/3214692
Reviewed-by: Chia-I Wu <olv@google.com>
Commit-Queue: Yiwei Zhang <zzyiwei@chromium.org>
Tested-by: Yiwei Zhang <zzyiwei@chromium.org>
3 files changed