i965/fs: Set up the builder execution size explicitly in opt_sampler_eot().

i965/fs: Set up the builder execution size explicitly in opt_sampler_eot().

opt_sampler_eot() was relying on the default builder to have the same
width as the sampler and FB write opcodes it was eliminating, the
channel selects didn't matter because the builder was only being used
to allocate registers, no new instructions were being emitted with it.
A future commit will change the width of the default builder what will
break this assumption, so initialize it explicitly here.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>