radeon/llvm: enable unsafe math for graphics shaders

radeon/llvm: enable unsafe math for graphics shaders

Accuracy of some operations was recently improved in the R600 backend,
at the cost of slower code. This is required for compute shaders,
but not for graphics shaders. Add unsafe-fp-math hint to make LLVM
generate faster but possibly less accurate code.

Piglit didn't indicate any regressions.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>