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/gbm_helpers.h b/gbm_helpers.h
index 48ff391..4d625c5 100644
--- a/gbm_helpers.h
+++ b/gbm_helpers.h
@@ -7,6 +7,6 @@
 #ifndef GBM_HELPERS_H
 #define GBM_HELPERS_H
 
-uint64_t gbm_convert_flags(uint32_t flags);
+uint64_t gbm_convert_usage(uint32_t usage);
 
 #endif