gallium/draw: Fix intermittent failure to bind new geometry shaders.

If you deleted your old GS and created a new one, then it would
occasionally skip binding the new GS because the token pointers were
equal.  Clear the current token pointer in the machine when we're deleting
its token.

Cc: mesa-stable
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8277>
(cherry picked from commit 969f50ddcfcf88f641779942210f33519761a08f)
2 files changed