minigbm: make drv_add_combinations return nothing

minigbm: make drv_add_combinations return nothing

Two reasons for this:

	1) We can use drv_array for driver combinations (see next patch)
	2) It's less verbose.

BUG=chromium:764871
TEST=gbmtest passes

Change-Id: I39bea6e2e9e4c76d2ca78566926a79bdc17f11d0
Reviewed-on: https://chromium-review.googlesource.com/758148
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>