minigbm: unmap right before buffer destruction

minigbm: unmap right before buffer destruction

Regardless of the allocator, we want to make sure any mappings
we create are eventually freed. This patch adds logic to
drv_bo_destroy to free any mappings before the GEM close
ioctl is called.

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

Change-Id: I8f4edb4bc01ff6e1d71a60e0309e77e9fc3840f4
Reviewed-on: https://chromium-review.googlesource.com/441916
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Joe Kniss <djmk@google.com>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>