mesa: do not build nouveau NVIDIA gallium-llvm driver if not requested

mesa: do not build nouveau NVIDIA gallium-llvm driver if not requested

The Nouveau driver is used for NVIDIA GPUs and thus doesn't really need
to be always enabled when building gallium-llvm drivers.

So let's guard it with a nouveau PACKAGECONFIG.

The only intended change is nouveau not being build on target mesa on
non-x86/i686 machines when building gallium-llvm drivers as I assume
NVIDIA GPUs aren't that common on other CPU architectures (tegra already
bringing in the nouveau driver should handle the NVIDIA SoCs that would
benefit from nouveau driver and thus are not impacted by this change).

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>