drm-tests: use gbm_bo_get_fd in null_platform_test

Originally, drmPrimeHandleToFD was used directly by null_platform_test,
but this let slip a bug in minigbm's implementation of gbm_bo_get_fd.
Now null_platform_test uses more of minigbm's features which mirrors how
Chrome uses minigbm.

BUG=None
TEST=null_platform_test

Change-Id: I7bff866f9317f7f2c8e25169bbc6487eef21df15
Reviewed-on: https://chromium-review.googlesource.com/239965
Reviewed-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Queue: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
1 file changed