minigbm: refactor driver helpers

1. frontends access the driver via drv.h only
2. the renamed drv_helpers and drv_array_helpers are for driver only
3. remove extern "C" from drv_helpers.h given not exposed to gralloc
4. remove all redundant includes for those helpers

BUG=b:199524294
TEST=CQ and gralloc builds on aosp

Change-Id: I3f4d33076a6a8161804f1b7c26950ff5496507e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/3195651
Tested-by: Yiwei Zhang <zzyiwei@chromium.org>
Reviewed-by: Chia-I Wu <olv@google.com>
Commit-Queue: Yiwei Zhang <zzyiwei@chromium.org>
diff --git a/dri.c b/dri.c
index f89bbed..8b55c32 100644
--- a/dri.c
+++ b/dri.c
@@ -18,8 +18,8 @@
 #include <xf86drm.h>
 
 #include "dri.h"
+#include "drv_helpers.h"
 #include "drv_priv.h"
-#include "helpers.h"
 #include "util.h"
 
 static const struct {