minigbm: Add YV12 for i915

minigbm: Add YV12 for i915

At the moment, the ArcCodec produces YV12 on Intel.  The gralloc
module should support it, atleast temporarily until UYVY hardware
overlays are enabled on CrOS.

TEST=ran graphics_Gbm, tested Chrome boots on Cyan
BUG=b/29335168, chromium:616275
CQ-DEPEND=CL:372359

Change-Id: I27c888d3467aa89e8ca48b22523cbc76973aa314
Reviewed-on: https://chromium-review.googlesource.com/373048
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>