minigbm: factor out metadata from struct bo

minigbm: factor out metadata from struct bo

Generated using coccinelle:

@@
struct bo *B;
@@

- B->width
+ B->width

BUG=chromium:924405
TEST=compile

Change-Id: I4da1731a650d198ce7f2bda3031a47b2f9c3041c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/1815566
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>