minigbm: quick and dirty implementation of gbm_bo_map/gbm_bo_unmap

minigbm: quick and dirty implementation of gbm_bo_map/gbm_bo_unmap

We want to add gbm_bo_map/gbm_bo_unmap entry points so certain tests
and Chrome can use driver specific map logic.  This is based on the
upstream entry points in Mesa, with the addition of a plane parameter.

Currently, we just map the entire buffer and don't attempt to do partial
mappings or use the map flags.  We should do this in the future...

BUG=chromium:653284
TEST=minigbm builds

Change-Id: I0423c10c55bab8fac6d6d7c6a699ab71b43aa61b
Reviewed-on: https://chromium-review.googlesource.com/393927
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>