minigbm: virtio-gpu: remove NV12 as texture source format

minigbm: virtio-gpu: remove NV12 as texture source format

This breaks the Ozone EGL unit tests.  This should still work for
Vivid Camera testing.

BUG=chromium:970923
TEST=ozone EGL unit test

Change-Id: I850584f926a7bd1cdd4dbc749d446ba775363e2a
Reviewed-on: https://chromium-review.googlesource.com/1646884
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>