minigbm: Fix switch statements

minigbm: Fix switch statements

Our switch statements were inconsistent.  Let's use kernel
style switch statements since we already follow many kernel
coding guidelines in this repo (8-space tabs, mostly 80
line limit).

Change-Id: I2b6ccc2369160bf49bbdf6479aebec7dada2e2fa
Reviewed-on: https://chromium-review.googlesource.com/377883
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>