softpipe: fix unreliable FS variant binding bug

softpipe: fix unreliable FS variant binding bug

In exec_prepare() we were comparing pointers to see if the fragment
shader variant had changed before calling tgsi_exec_machine_bind_shader().
This didn't work reliably when there was a lot of shader token malloc/
freeing going on because the memory might get reused.
Instead, bind the shader variant during regular state validation.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=40404
(fixes a couple of piglit's glsl-max-varyings test)

Note: This is a candidate for the stable branches.