minigbm: re-const-ify driver backends

minigbm driver creation needs to be re-entrant (see CL:674528). Let's
re-constify to make this behavior explicit.

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

Change-Id: I037966199d4aa6de60432127e10fea1fb602694b
Reviewed-on: https://chromium-review.googlesource.com/758142
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
diff --git a/i915.c b/i915.c
index 5869a59..2dc2484 100644
--- a/i915.c
+++ b/i915.c
@@ -547,7 +547,7 @@
 	}
 }
 
-struct backend backend_i915 = {
+const struct backend backend_i915 = {
 	.name = "i915",
 	.init = i915_init,
 	.close = i915_close,