minigbm: align multi-planar formats to 16

minigbm: align multi-planar formats to 16

libva 1.8.3 makes it a hard requirement that multi-planar surfaces are
vertically aligned to 16. Enforce this rule at allocation time.

BUG=b:65084476
TEST=checked that b:65084476 was not reproducible with this patch and
that all multi-planar surfaces are aligned to 16.

Change-Id: Id31055df81ba43130f3f45282db104753fb6e700
Reviewed-on: https://chromium-review.googlesource.com/642429
Commit-Ready: Alexandre Courbot <acourbot@chromium.org>
Tested-by: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>