minigbm: use struct vma for (*bo_map)/(*bo_unmap) callbacks

minigbm: use struct vma for (*bo_map)/(*bo_unmap) callbacks

This sets better expectations for what we expect from the
backends.

BUG=chromium:764871
TEST=mmap_test

Change-Id: I7fb815b58fae8e9fbd73bf7c0263c7db44488844
Reviewed-on: https://chromium-review.googlesource.com/770519
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>