minigbm: standardize naming of buffer creation flags

We use the terms "flags" and "usage" interchangeably in this repo,
since they essentally mean the same thing.  However, let's be a
little more consistent since it's kind of confusing, especially
when buffer map flags come to play. Let's:

	- refer to everything in the drv_* layer as use_flags
	- refer to everything in the gbm/gralloc layers as
	  usages

BUG=chromium:764871
TEST=emerge-eve {arc-cros-gralloc, minigbm}

Change-Id: If987d72369b895f38cde87e50ce1080f78f2a084
Reviewed-on: https://chromium-review.googlesource.com/691423
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
diff --git a/vc4.c b/vc4.c
index dd85bc3..04e0ad9 100644
--- a/vc4.c
+++ b/vc4.c
@@ -31,7 +31,7 @@
 }
 
 static int vc4_bo_create(struct bo *bo, uint32_t width, uint32_t height, uint32_t format,
-			 uint64_t flags)
+			 uint64_t use_flags)
 {
 	int ret;
 	size_t plane;