minigbm: Use the stride value returned by mapImage

mapImage can return a different stride value. This value must be
used for the mapped address.

BUG=b:38152101 b:79346377
TEST=graphics_Sanity autotest passed

Change-Id: Ie0eb716291366ae6a047d704f66d24ec41738713
Signed-off-by: Satyajit Sahu <satyajit.sahu@amd.com>
Reviewed-on: https://chromium-review.googlesource.com/1041369
Commit-Ready: Bernie Thompson <bhthompson@chromium.org>
Tested-by: Drew Davenport <ddavenport@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
4 files changed