minigbm: add (*bo_flush) implementations

minigbm: add (*bo_flush) implementations

Let's take the flush and writeback steps in the driver unmap
functions and move them to the (*bo_flush) callback.

BUG=chromium:764871
TEST=gbmtest, mmap_test -g on eve

Change-Id: Iecccafc25642b120bc84f2d1b8538303c8ce36eb
Reviewed-on: https://chromium-review.googlesource.com/668219
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>