softpipe: fix up FS variant unbinding / deletion

softpipe: fix up FS variant unbinding / deletion

The old call to tgsi_exec_machine_bind_shader() in
softpipe_delete_fs_state() was never called since the shader's original
tokens are never passed to the tgsi interpreter (only shader _variant_
tokens are).  Now, unbind the variant's tokens from the tgsi interpreter
when we free the variant.

This doesn't fix any known bugs but it's the right thing to do.

Note: This is a candidate for the stable branches.
(cherry picked from commit fddcc67f5cb41f70d1d11827eb9190a5caec13c5)