minigbm: introduce test allocation

minigbm: introduce test allocation

This change introduces a GBM_TEST_ALLOC flag to minigbm, which allows
for the creation of fake buffers that can be used to determine buffer
metadata without actually allocating a full buffer. The new flag is
supported by the i915 backends. This flag also alleviates the need to
cache buffers when virtio_gpu queries metadata properties.

BUG=b:145994510
TEST=play youtube with arcvm demo image plus this and virgl change

Change-Id: I9c6819aa3b5b674e4bb33b0656f2a9f155b0884e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/1980688
Tested-by: David Stevens <stevensd@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Commit-Queue: David Stevens <stevensd@chromium.org>