minigbm: vgem: align to llvm tile size

minigbm: vgem: align to llvm tile size

We're planning on using Mesa with llvmpipe when running
the Android container in a virtual machine. It's faster
than softpipe. llvmpipe requires alignment to a 64-byte
tile size, so add it here.  This is the analogue to
norvez@'s drm_gralloc commiti (ag/1599269).

BUG=b:28802929, chromium:616275
TEST=run arc-cros-gralloc in QEMU

CQ-DEPEND=CL:414537

Change-Id: If4160d376447ce218df3bc410a8427b284de74fd
Reviewed-on: https://chromium-review.googlesource.com/416291
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>