minigbm: Fix incorrect plane import size

We were incorrectly determining the plane size for buffers with
vertical subsampling.  Let's add it here.

BUG=chromium:618516
TEST=graphics_Gbm still passes

Change-Id: I47f3c01dfd2539fc669a017ecb497d754d29de9f
Reviewed-on: https://chromium-review.googlesource.com/406485
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
5 files changed