minigbm: Fill sizes[0] in gbm_bo_import

minigbm: Fill sizes[0] in gbm_bo_import

We need to give useable sizes to imported buffers, not just the ones
we create.

BUG=chromium:592681
TEST=run the test on lumpy

Change-Id: Ife99988600607e2f4a56bbad37b48c21829701dd
Reviewed-on: https://chromium-review.googlesource.com/332448
Commit-Ready: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Ilja Friedel <ihf@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>