minigbm: vgem: allow driver creation without planes resources

minigbm: vgem: allow driver creation without planes resources

With CL:405019, when drmModeGetPlaneResources failed, creation of
the driver failed. This was an issue for our vgem backend, which
we use to run ARC++ with QEMU. Let's not add KMS flags for vgem
since we know that doesn't work.

BUG=b:31942635
TEST=arc-cos-gralloc with vgem works

CQ-DEPEND=CL:415442

Change-Id: I33a812ab9a24e2e35a47d28f720c4cc2e83eb8f0
Reviewed-on: https://chromium-review.googlesource.com/416290
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>