minigbm: use drv_array for mappings

minigbm: use drv_array for mappings

Let's start allowing multiple mappings of the same buffer when
different map flags are passed in.

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

Change-Id: I4eb0b6f4c3572a92001696c2720d5a5f7d9d73a4
Reviewed-on: https://chromium-review.googlesource.com/758146
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>