minigbm: cros_gralloc: fix initialization race condition

minigbm: cros_gralloc: fix initialization race condition

On Android M, since we don't hold a lock during cros_gralloc_driver::init(),
another thread would sometimes access data before the driver was initialized.
This would lead to dEQP crashes. We don't experience this issue on Android
N since the framework has it's own layer of locks in the Gralloc1On0Adapter.

This patch makes the module initialization during (*registerBuffer) and
gralloc_open re-entrant.

BUG=b:63511976
TEST=run cts --package com.drawelements.deqp.gles2 on veyron_tiger has no
     crashes

Change-Id: I2b72f2f8ed5e4a5afbacb291ed8cd928beb2a3b3
Reviewed-on: https://chromium-review.googlesource.com/597015
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>