minigbm: fix stride logic

minigbm: fix stride logic

We shouldn't round up to the nearest byte. This way we can represent
formats like Y41P.  Makes no difference currently since
drv_bpp_from_format only returns numbers divisible by eight.

Change-Id: I57bcab383f1661ea11e9aa1d30e18750574143e2
Reviewed-on: https://chromium-review.googlesource.com/462478
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>