minigbm: pass in buffer creation flags of the right size

The buffer creation flags are uint64_t, not uint32_t.

BUG=none
TEST=emerge-eve {minigbm, arc-cros-gralloc}

Change-Id: Id94911c5e19c00d47f2fbdca570a6a08fc92b82e
Reviewed-on: https://chromium-review.googlesource.com/691422
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>