Revert "minigbm: remove vgem backend"

Revert "minigbm: remove vgem backend"

This reverts commit d014ed5b4c12e7c806260e2ae304accfcd6c73aa.

Reason for revert: broke video tests on rockchip devices b/141093122
Bug: 141278896
Bug: 141093122

Original change's description:
> minigbm: remove vgem backend
>
> It's not used anymore, or atleast shouldn't be.
>
> BUG=none
> TEST=CQ will test
>
> Change-Id: Ib9232a7704bd39e59a8e2a5bc2ece62c5dd8e9c1
> Reviewed-on: https://chromium-review.googlesource.com/1643676
> Tested-by: Gurchetan Singh <gurchetansingh@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: Stéphane Marchesin <marcheu@chromium.org>

Bug: none
Change-Id: I6645e9b32ebc6dcd97d126cc40523f516a201a05
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/1812540
Reviewed-by: Kazuhiro Inaba <kinaba@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Kazuhiro Inaba <kinaba@chromium.org>
Auto-Submit: Kazuhiro Inaba <kinaba@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>