llvmpipe: fix race between draw and setting fragment shader.

llvmpipe: fix race between draw and setting fragment shader.

There is a race with u_blitter shaders + pipeline shaders (aaline/aapoint)
where the draw bind can cause a pipeline flush which can use bind_fs_state to
 be reenters and llvmpipe->fs gets the wrong value. Fix this by only
setting the llvmpipe->fs value after the draw binding is complete.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4122>