minigbm: Refactored minigbm on top a private API

minigbm: Refactored minigbm on top a private API

We would like to reuse the same set of drivers for ChromeOS (with
a minigbm frontend) and Android (with a gralloc frontend).  Since
we don't want to pollute the gbm API with gralloc formats and usages,
we can refactor minigbm on top a private API that will be a superset
of gbm and gralloc.  This change redirects gbm calls to the
private API.

TEST=Ran graphics_Gbm on minnie and cyan, and checked if Chrome boots.
BUG=chromium:616275

CQ-DEPEND=CL:367791

Change-Id: I50d10f9d6c7ea936b0d76c5299a58d948939fdf9
Reviewed-on: https://chromium-review.googlesource.com/367780
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>