minigbm: virtio-gpu: stride == level

minigbm: virtio-gpu: stride == level

Unfortunately, the kernel doesn't actually pass the guest layer_stride
and guest stride to the host (compare virtio_gpu.h and virtgpu_drm.h).
We can use the level to work around this.

BUG=b:132939420
TEST=none

Change-Id: I96927b22fae2662a3a37e5191b3dfdf86c99dd84
Reviewed-on: https://chromium-review.googlesource.com/1683087
Tested-by: David Stevens <stevensd@chromium.org>
Commit-Ready: David Stevens <stevensd@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: David Stevens <stevensd@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>