Build fix for older libdrm versions.

Build fix for older libdrm versions.

When building against an older libdrm, the DRM_FORMAT_MOD_LINEAR enum
will be missing. It's OK to use DRM_FORMAT_MOD_NONE instead, because
this older libdrm has limited DRM_FORMAT_MOD_xx support and not all of
the backends actually need it.

Change-Id: I0c289c50de4ad1fe5a9b0848961f5bcb75bd4bf8
Reviewed-on: https://chromium-review.googlesource.com/971358
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>