gallivm: fix buffer_type for LLVM 15
Fixes deqp-vk: /home/airlied/devel/llvm-project/llvm/include/llvm/IR/Instructions.h:961: static llvm::GetElementPtrInst* llvm::GetElementPtrInst::Create(llvm::Type*, llvm::Value*, llvm::ArrayRefllvm::Value*, const llvm::Twine&, llvm::Instruction*): Assertion `cast(Ptr->getType()->getScalarType()) ->isOpaqueOrPointeeTypeMatches(PointeeType)' failed.
Fixes: 1b8c4931 ("gallivm: fixes for LLVM-15 opaque pointers in lp_bld_jit_types.c")