i965/gs: Set GS prog_data to NULL if there is no GS program.

i965/gs: Set GS prog_data to NULL if there is no GS program.

The previous commit fixes a bug wherein we would incorrectly refer to
stale geometry shader prog_data when no geometry shader was active.

This patch reduces the likelihood of that sort of bug occurring in the
future by setting prog_data to NULL whenever there is no GS program.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>