Add GBM_BO_USE_HW_VIDEO_ENCODER use flag

Add GBM_BO_USE_HW_VIDEO_ENCODER use flag

Chrome needs to allocate a linear buffer that a hardware video encoder can read
and cpu can read and write. There is no use flag in gbm that specifies the
former. This CL introduces a new use flag for that.

BUG=b:138703716
TEST=None

Change-Id: Ied0321914a366294a47e4fc5c2a8f08ee0351bd8
Reviewed-on: https://chromium-review.googlesource.com/1728729
Tested-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>