r600g: Disable LLVM by default at runtime for graphics

r600g: Disable LLVM by default at runtime for graphics

For graphics, the LLVM compiler backend currently has many shortcomings
compared to the non-LLVM one. E.g. it can't handle geometry shaders yet,
but that's just the tip of the iceberg.

So building Mesa with --enable-r600-llvm-compiler is currently not
recommended for anyone who doesn't want to work on fixing those issues.
However, for protection of users who end up enabling it anyway for some
reason, let's disable the LLVM backend at runtime by default. It can be
enabled with the environment variable R600_DEBUG=llvm.

Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>