minigbm: Revert "minigbm: flush buffer instead of unmapping"

minigbm: Revert "minigbm: flush buffer instead of unmapping"

It looks like munmap is probably necessary for i915 and serves
some important function. Let's revert until we can figure
out what's going on.

BUG=b:67073097
TEST=android.view.cts.SurfaceViewSyncTests#testVideoSurfaceViewCornerCoverage

This reverts commit 254dbb19c49fad4ab4f28b2d1f2982bd10bb7c0b.

Change-Id: Idac50930f9f13b4867110a31fadb1a2547d05d07
Reviewed-on: https://chromium-review.googlesource.com/699802
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>