softpipe: allow softpipe to set shader params depending on runtime llvm (v3)

softpipe: allow softpipe to set shader params depending on runtime llvm (v3)

If draw isn't using llvm we can support vertex texture and integers,
These will be fixed up later, but for now allow this check to happen
at run-time.

v2: since 3e22c7a25321554a32fa6254485912fd53deff3a we can ask draw for a non-llvm
context. Just track if ask and set the vars accordingly. This probably isn't perfect but should cover the cases we care about.

v3: use debug option, restructure to store in screen, as suggested by Jakob.

Signed-off-by: Dave Airlie <airlied@redhat.com>