minigbm: i915: Use NV12 as the main YUV format, including camera output

minigbm: i915: Use NV12 as the main YUV format, including camera output

On SKL and newer, the display controller supports NV12 natively for
overlays. Moreover it's the only output format supported by the camera
subsystem. Since it shouldn't matter for other uses cases if we use NV12
or YVU420, let's just switch everything to NV12 for simplicity.

BUG=b:32077885
TEST=Play a video in Android YouTube app on Cyan and Reef

Change-Id: I5cfcdec2b0e2020a3ae1e8364b60df3585838807
Reviewed-on: https://chromium-review.googlesource.com/544489
Commit-Ready: Owen Lin <owenlin@chromium.org>
Tested-by: Owen Lin <owenlin@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>