minigbm: add vgem backend

minigbm: add vgem backend

When running x86 ARC++ images on a virtual machine, we use
the vgem driver.  We should support it here too.

BUG=chromium:616275
TEST=container boots when running QEMU with cros_gralloc.  Note
videos (i.e, YUV formats) don't work on QEMU, but they never did
with gralloc_drm either.

CQ-DEPEND=CL:392548

Change-Id: If9f6b8035f2a88068980bede5121ef4599b37272
Reviewed-on: https://chromium-review.googlesource.com/392549
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>