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>
diff --git a/drv.h b/drv.h
index 4b70d58..9f8330e 100644
--- a/drv.h
+++ b/drv.h
@@ -212,6 +212,13 @@
 int
 drv_stride_from_format(uint32_t format, uint32_t width, size_t plane);
 
+size_t
+drv_num_planes_from_format(uint32_t format);
+
+uint32_t
+drv_size_from_format(drv_format_t format, uint32_t stride, uint32_t height,
+		     size_t plane);
+
 uint32_t
 drv_num_buffers_per_bo(struct bo *bo);