minigbm: virtio-gpu: use NV12 as flexible media format

minigbm: virtio-gpu: use NV12 as flexible media format

All of our future drivers (including MTK_MT8183) seem to
prefer NV12 as their flexible media format, so advertise that.

In the unlikely event that changes, we can set either use the
capabilities or resource create versioning (a planned feature).

This has the benefit of keeping Android flexible formats in
Android userspace.

BUG=b:132939420
TEST=compile

Change-Id: I486f7be23f8431aa8bbaea89dca7c1304536e9ac
Reviewed-on: https://chromium-review.googlesource.com/1644464
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Lepton Wu <lepton@chromium.org>