mesa: do not build r300 AMD gallium-llvm driver if not requested

mesa: do not build r300 AMD gallium-llvm driver if not requested

It doesn't make much sense to always build the r300 AMD gallium LLVM
driver as that's HW-specific.

Instead, let's guard it with the amd PACKAGECONFIG.

This will only disable R300 for target mesa on non-x86/i686 machines
where gallium-llvm PACKAGECONFIG is selected but not amd, otherwise
behavior is left unchanged.

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