i965/fs,vec4: Get rid of the sanity_param_count

i965/fs,vec4: Get rid of the sanity_param_count

It doesn't exist for anything other than an assert that, as far as I can
tell, isn't possible to trip.  Soon, we will remove prog from the visitor
entirely and this will become even more impossible to hit.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>