minigbm: respect map_flags in all cases

minigbm: respect map_flags in all cases

When the map flags do not feature BO_TRANSFER_WRITE, we should
not copy back any shadow buffers we maintain.

BUG=b:67434931
TEST=android.view.cts.SurfaceViewSyncTests on Mediatek/Rockchip

Change-Id: I7078bfc5a8d770a52949d43ea68efc4a870e9227
Reviewed-on: https://chromium-review.googlesource.com/703875
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Commit-Queue: Gurchetan Singh <gurchetansingh@chromium.org>