minigbm: One buffer for NV12

minigbm: One buffer for NV12

Some of our Android framework code expects one fd per format.  The gbm
API allows it, so let's go with this representation.  Also fixed a bug
that comes up when bo->num_planes = 2, but the handles are the same.

BUG=chromium:616275
TEST=gbmtest, plane_test

Change-Id: Iec0e6319d144941a73c26497af2078112d5e4e64
Reviewed-on: https://chromium-review.googlesource.com/360905
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>