i965: Don't disable exec masking for sampler message sends.

i965: Don't disable exec masking for sampler message sends.

This was telling the sampler to do texture fetches for *all* channels
in the non-constant surface index case, what could have reduced
throughput unnecessarily when some of the channels were disabled by
control flow.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>