minigbm: Add dumb buffer 'nouveau' backend.

minigbm: Add dumb buffer 'nouveau' backend.

Adding a nouveau backend to minigbm can be useful to test ozone code
on our workstations.

This CL adds a nouveau backend that uses dumb buffers.
This is enough to run ozone_gl_unittests and ozone_demo on our
workstations.

BUG=None
TEST='Ran ozone_demo on my workstation.'

Change-Id: I4e690192c9d9944b9907e64253205a94605a76eb
Reviewed-on: https://chromium-review.googlesource.com/421606
Commit-Ready: David Reveman <reveman@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>