[mlir] Replace MLIR_ENABLE_CUDA_CONVERSIONS with LLVM_HAS_NVPTX_TARGET (#93008)

LLVM_HAS_NVPTX_TARGET is automatically set depending on whether NVPTX
was enabled when building LLVM. Use this instead of manually defining
MLIR_ENABLE_CUDA_CONVERSIONS (whose name is a bit misleading btw).