minigbm: rockchip: add NV12 back in as supported format

minigbm: rockchip: add NV12 back in as supported format

CL:448252 inadvertently removed this format.

BUG=none
TEST=play video via Youtube app

Change-Id: Ia57c66ffdd6f9ebb401c09dab5ea82eba2fe1893
Reviewed-on: https://chromium-review.googlesource.com/457756
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>