radeonsi: store group_size_variable in struct si_compute

radeonsi: store group_size_variable in struct si_compute

For compute shaders, we free the selector after the shader has been
compiled, so we need to save this bit somewhere else.  Also, make sure that
this type of bug cannot re-appear, by NULL-ing the selector pointer after
we're done with it.

This bug has been there since the feature was added, but was only exposed
in piglit arb_compute_variable_group_size-local-size by commit
9bfee7047b70cb0aa026ca9536465762f96cb2b1 (which is totally unrelated).

Cc: 13.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>