Use lseek() ISO gbm_bo_get_plane_size()

This CL follows up on a comment in http://crrev/c/2808787 and
stops using gbm_bo_get_plane_size(), using lseek() instead.
Since lseek() needs an fd and usually we start from a struct
gbm_bo, I made a local function in mmap.c (and inlined the
other callsite in mmap_test.c

BUG=b:183760490
TEST=emerge && deploy on volteer, mmap_test -b/-s/-d/-g working

Change-Id: Ife14d703da0c5d202cb6b51b1a34f7e333bac435
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/drm-tests/+/2815603
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Tested-by: Miguel Casas <mcasas@chromium.org>
2 files changed