draw/llvm: reduce memory usage

draw/llvm: reduce memory usage

Lets make draw_get_option_use_llvm function available unconditionally
and use it to avoid useless allocations when LLVM paths are active.
TGSI machine is never used when we're using LLVM.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>