minigbm: drv_bo_flush --> drv_bo_flush_or_unmap

minigbm: drv_bo_flush --> drv_bo_flush_or_unmap

There's no flush or invalidate mechnaism with the DRI interface,
so we'll have to support mapping and unmapping.

BUG=b:72972511
TEST=gbmtest passes in CQ

Change-Id: If3aef651f8549544ed4ed499e17f7f99c597974c
Reviewed-on: https://chromium-review.googlesource.com/990892
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Satyajit Sahu <satyajit.sahu@amd.com>
Reviewed-by: Joe Kniss <djmk@google.com>
Reviewed-by: Satyajit Sahu <satyajit.sahu@amd.com>