draw: do not use draw_get_option_use_llvm() inside draw execution paths

draw: do not use draw_get_option_use_llvm() inside draw execution paths

1c73e919a4b4dd79166d0633075990056f27fd28 made it possible to not allocate
the tgsi machine if llvm was used. However, draw_get_option_use_llvm() is
not reliable after draw context creation, since drivers can explicitly
request a non-llvm draw context even if draw_get_option_use_llvm() would
return true (and softpipe does just that) which leads to crashes.
Thus use draw->llvm to determine if we're using llvm or not instead (and
make draw->llvm available even if HAVE_LLVM is false so we don't have to put
even more ifdefs).

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>