draw: fix base instance handling in llvm path

draw: fix base instance handling in llvm path

The base instance needs to be passed to the jited function, otherwise the
instanced data fetch will only work with the same start instance when the
jit function was created (and baking that into the key instead is not a viable
option).
This fixes piglit arb_base_instance-drawarrays (modulo some unrelated
core/compat context trouble I get for the test).
And fix the pipe cap bit in llvmpipe for it now that it actually works (it
already worked for softpipe).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>